metadata: Create compact LMDB snapshots

See #1006

LMDB files never shrink, so we can end up with a large database that
contains a smaller amount of actual data.

Compacting the snapshots is an easy win: it will write faster to disk,
take less space, and if needed you can reimport an already-compacted
snapshot as the main database.

src/db/lmdb_adapter.rs

@@ -109,7 +109,7 @@ impl IDb for LmdbDb {
 		let mut path = to.clone();
 		path.push("data.mdb");
 		self.db
-			.copy_to_path(path, heed::CompactionOption::Disabled)?;
+			.copy_to_path(path, heed::CompactionOption::Enabled)?;
 		Ok(())
 	}