Review use of threadsafety in sqlite

[tcompa]

Ref:

• https://fastapi.tiangolo.com/tutorial/sql-databases/#create-the-sqlalchemy-engine
• https://docs.python.org/3/library/sqlite3.html#sqlite3.threadsafety

[tcompa]

On my machine:

>>> import sqlite3
>>> sqlite3.threadsafety
1
>>> 

which maps to SQLITE_THREADSAFE=1, that is "Threads may share the module, but not connections". More explicitly:

Multi-thread: In this mode, SQLite can be safely used by multiple threads provided that no single database connection is used simultaneously in two or more threads.

As of #664, when using sqlite we fall back onto StaticPool: "A Pool of exactly one connection, used for all requests.". This seems to solve (or at least mitigate) known issues (as in #661).

It is possible that switching to a different sqlite3 version, with higher threadsafety (coming from a different SQLITE_THREADSAFE of the underlying library), would also solve the known issues. We did not test this idea, because in practice it would introduce a complex set of dependencies.

Closing for now.