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/k2v/item_table.rs

@@ -2,6 +2,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::sync::Arc;
 
+use garage_db as db;
 use garage_util::data::*;
 
 use garage_table::crdt::*;
@@ -221,7 +222,12 @@ impl TableSchema for K2VItemTable {
 	type E = K2VItem;
 	type Filter = ItemFilter;
 
-	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<()> {
 		// 1. Count
 		let (old_entries, old_conflicts, old_values, old_bytes) = match old {
 			None => (0, 0, 0, 0),
@@ -239,7 +245,8 @@ impl TableSchema for K2VItemTable {
 			.map(|e| &e.partition.partition_key)
 			.unwrap_or_else(|| &new.unwrap().partition.partition_key);
 
-		if let Err(e) = self.counter_table.count(
+		let counter_res = self.counter_table.count(
+			tx,
 			&count_pk,
 			count_sk,
 			&[
@@ -248,14 +255,23 @@ impl TableSchema for K2VItemTable {
 				(VALUES, new_values - old_values),
 				(BYTES, new_bytes - old_bytes),
 			],
-		) {
-			error!("Could not update K2V counter for bucket {:?} partition {}; counts will now be inconsistent. {}", count_pk, count_sk, e);
+		);
+		if let Err(e) = db::unabort(counter_res)? {
+			// This result can be returned by `counter_table.count()` for instance
+			// if messagepack serialization or deserialization fails at some step.
+			// Warn admin but ignore this error for now, that's all we can do.
+			error!(
+				"Unable to update K2V item counter for bucket {:?} partition {}: {}. Index values will be wrong!",
+				count_pk, count_sk, e
+			);
 		}
 
 		// 2. Notify
 		if let Some(new_ent) = new {
 			self.subscriptions.notify(new_ent);
 		}
+
+		Ok(())
 	}
 
 	#[allow(clippy::nonminimal_bool)]