feat: replace leveldb with sqlite3

[charles-cooper]

What I did

for our use case, sqlite should work just about as well as leveldb. (it may be slightly slower, but requests to sqlite3 should be in the 1us-10us range).

leveldb on python has two drawbacks:

• poor cross-platform support (Add github actions CI/CD for building cross-platform(Linux, MacOS and Windows) wheels for leveldb 1.23 and snappy 1.1.9 wbolster/plyvel#129)
• the real dealbreaker is no multiprocessing support, as leveldb only allows single-process access (cf. Concurrency support for multiple processes (1 exclusive initializer / n readers) google/leveldb#182).

How I did it

implement a "SqliteDB" with ttl for cached keys.

How to verify it

Description for the changelog

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