Move in-memory bookkeeping into SQLite

[jeromegn]

Corrosion track bookkeeping in memory, as well as in SQLite. It uses more efficient data structures in memory. This makes accessing the state of Corrosion out-of-band very fast and easy.

The reason for its existence is: finding "gaps" in SQLite tables is hard to optimize and the queries are therefore resource intensive. Being able to quickly figure out if Corrosion has "seen" a version, without a DB lookup, before checking out a write connection from the pool is efficient.

Unfortunately, it also comes with a lot of downsides:

• The in-memory bookkeeping structure needs to be protected via locking as it can be used from any threads.
• Since we're locking, we can't easily bundle up many change insertions into one transaction as we require locking on a structure before inserting.
• We have to periodically compact the in-memory representation based on what's in the database to keep memory usage low

It might be possible to get rid of it entirely or design it in a way that it's not a requirement for inserts. Could likely be achieved by devising queries that looks at bookkeeping as Corrosion tries to insert data.