Draft: Threadpool refactor fsds

[elcritch]

Refactor sqliteds into a synchronous backend. The ThreadDatastore was then refactored to use the synchronous sqlite backend. The ThreadDatastore is based on #48 but with simplifications due to not needing to handle Futures on the backend.

The FsDs should now be pretty easy to refactor by re-using the design for SqliteDs.

Other changes:

• SqliteDs should be fully thread-safe now barring errors
• SqliteDs is now fully generic and can use keys of string | KeyId and data as seq[byte] | DataBuffer
• ThreadDatastore is about ~20% shorter than previously since there's no futures on the backend
• ThreadDatastore seems to be about ~10% faster, but just based off timing tests
• Queries run on a single thread simplifying key conflicts and preventing queries from swamping the system

Some todos:

• fix potential FsBackend race conditions