Sync commit

[levkk]

Description

Implement synchronous commit in PgDog. Hold query until the transaction is flushed to all replicas. Requires logical replication integration likely, to confirm table changes.

[levkk]

I'm thinking we create a replication slot on the primary and replicas (pg 16+) and assert that primary's latest LSN is >= all replicas before we tell the client the query is complete. Disconnect from the server while waiting, so this should have minimal impact on the DB.