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/db/bin/convert.rs

@@ -0,0 +1,76 @@
+use std::path::PathBuf;
+
+use garage_db::*;
+
+use clap::Parser;
+
+/// K2V command line interface
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+struct Args {
+	/// Input DB path
+	#[clap(short = 'i')]
+	input_path: PathBuf,
+	/// Input DB engine
+	#[clap(short = 'a')]
+	input_engine: String,
+
+	/// Output DB path
+	#[clap(short = 'o')]
+	output_path: PathBuf,
+	/// Output DB engine
+	#[clap(short = 'b')]
+	output_engine: String,
+}
+
+fn main() {
+	let args = Args::parse();
+	pretty_env_logger::init();
+
+	match do_conversion(args) {
+		Ok(()) => println!("Success!"),
+		Err(e) => eprintln!("Error: {}", e),
+	}
+}
+
+fn do_conversion(args: Args) -> Result<()> {
+	let input = open_db(args.input_path, args.input_engine)?;
+	let output = open_db(args.output_path, args.output_engine)?;
+	output.import(&input)?;
+	Ok(())
+}
+
+fn open_db(path: PathBuf, engine: String) -> Result<Db> {
+	match engine.as_str() {
+		"sled" => {
+			let db = sled_adapter::sled::Config::default().path(&path).open()?;
+			Ok(sled_adapter::SledDb::init(db))
+		}
+		"sqlite" | "sqlite3" | "rusqlite" => {
+			let db = sqlite_adapter::rusqlite::Connection::open(&path)?;
+			Ok(sqlite_adapter::SqliteDb::init(db))
+		}
+		"lmdb" | "heed" => {
+			std::fs::create_dir_all(&path).map_err(|e| {
+				Error(format!("Unable to create LMDB data directory: {}", e).into())
+			})?;
+
+			let map_size = if u32::MAX as usize == usize::MAX {
+				eprintln!(
+					"LMDB is not recommended on 32-bit systems, database size will be limited"
+				);
+				1usize << 30 // 1GB for 32-bit systems
+			} else {
+				1usize << 40 // 1TB for 64-bit systems
+			};
+
+			let db = lmdb_adapter::heed::EnvOpenOptions::new()
+				.max_dbs(100)
+				.map_size(map_size)
+				.open(&path)
+				.unwrap();
+			Ok(lmdb_adapter::LmdbDb::init(db))
+		}
+		e => Err(Error(format!("Invalid DB engine: {}", e).into())),
+	}
+}