Abstract database behind generic interface and implement alternative drivers (#322)

- [x] Design interface
- [x] Implement Sled backend
  - [x] Re-implement the SledCountedTree hack ~~on Sled backend~~ on all backends (i.e. over the abstraction)
- [x] Convert Garage code to use generic interface
- [x] Proof-read converted Garage code
- [ ] Test everything well
- [x] Implement sqlite backend
- [x] Implement LMDB backend
- [ ] (Implement Persy backend?)
- [ ] (Implement other backends? (like RocksDB, ...))
- [x] Implement backend choice in config file and garage server module
- [x] Add CLI for converting between DB formats
- Exploit the new interface to put more things in transactions
  - [x] `.updated()` trigger on Garage tables

Fix #284

**Bugs**

- [x] When exporting sqlite, trees iterate empty??
- [x] LMDB doesn't work

**Known issues for various back-ends**

- Sled:
  - Eats all my RAM and also all my disk space
  - `.len()` has to traverse the whole table
  - Is actually quite slow on some operations
  - And is actually pretty bad code...
- Sqlite:
  - Requires a lock to be taken on all operations. The lock is also taken when iterating on a table with `.iter()`, and the lock isn't released until the iterator is dropped. This means that we must be VERY carefull to not do anything else inside a `.iter()` loop or else we will have a deadlock! Most such cases have been eliminated from the Garage codebase, but there might still be some that remain. If your Garage-over-Sqlite seems to hang/freeze, this is the reason.
  - (adapter uses a bunch of unsafe code)
- Heed (LMDB):
  - Not suited for 32-bit machines as it has to map the whole DB in memory.
  - (adpater uses a tiny bit of unsafe code)

**My recommendation:** avoid 32-bit machines and use LMDB as much as possible.

**Converting databases** is actually quite easy. For example from Sled to LMDB:

```bash
cd src/db
cargo run --features cli --bin convert -- -i path/to/garage/meta/db -a sled -o path/to/garage/meta/db.lmdb -b lmdb
```

Then, just add this to your `config.toml`:

```toml
db_engine = "lmdb"
```

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/322
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/model/s3/block_ref_table.rs

@@ -1,6 +1,8 @@
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::data::*;
 
 use garage_table::crdt::Crdt;
@@ -51,21 +53,22 @@ impl TableSchema for BlockRefTable {
 	type E = BlockRef;
 	type Filter = DeletedFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
-		#[allow(clippy::or_fun_call)]
-		let block = &old.or(new).unwrap().block;
+	fn updated(
+		&self,
+		tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
+		let block = old.or(new).unwrap().block;
 		let was_before = old.map(|x| !x.deleted.get()).unwrap_or(false);
 		let is_after = new.map(|x| !x.deleted.get()).unwrap_or(false);
 		if is_after && !was_before {
-			if let Err(e) = self.block_manager.block_incref(block) {
-				warn!("block_incref failed for block {:?}: {}", block, e);
-			}
+			self.block_manager.block_incref(tx, block)?;
 		}
 		if was_before && !is_after {
-			if let Err(e) = self.block_manager.block_decref(block) {
-				warn!("block_decref failed for block {:?}: {}", block, e);
-			}
+			self.block_manager.block_decref(tx, block)?;
 		}
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {

src/model/s3/object_table.rs

@@ -2,6 +2,8 @@ use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
@@ -232,7 +234,12 @@ impl TableSchema for ObjectTable {
 	type E = Object;
 	type Filter = ObjectFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
+	fn updated(
+		&self,
+		_tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
 		let version_table = self.version_table.clone();
 		let old = old.cloned();
 		let new = new.cloned();
@@ -259,7 +266,8 @@ impl TableSchema for ObjectTable {
 				}
 			}
 			Ok(())
-		})
+		});
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {

src/model/s3/version_table.rs

@@ -1,6 +1,8 @@
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
+use garage_db as db;
+
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
@@ -137,7 +139,12 @@ impl TableSchema for VersionTable {
 	type E = Version;
 	type Filter = DeletedFilter;
 
-	fn updated(&self, old: Option<&Self::E>, new: Option<&Self::E>) {
+	fn updated(
+		&self,
+		_tx: &mut db::Transaction,
+		old: Option<&Self::E>,
+		new: Option<&Self::E>,
+	) -> db::TxOpResult<()> {
 		let block_ref_table = self.block_ref_table.clone();
 		let old = old.cloned();
 		let new = new.cloned();
@@ -160,7 +167,9 @@ impl TableSchema for VersionTable {
 				}
 			}
 			Ok(())
-		})
+		});
+
+		Ok(())
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {