Add connection pool support

[tantaman]

wa-sqlite serializes reads that are on the same connection.

We should update the sqlite connectors to:

• have a single write connection (all writes are always serialized even at the sqlite level)
• have N read connections

To allow for parallel reads.

[tantaman]

https://github.com/tantaman/aphrodite/tree/connection-pooling

[tantaman]

49c6743

[tantaman]

Connection pooling isn't supported against in-memory db for node-sqlite3.

wa-sqlite still serializes queries even when multiple connections exist to the db. Maybe this can be resolved by opening a new worker per wa-sqlite connection.

We'll table connection pooling until perf becomes a larger issue.