Prefer transaction-level locks instead of session-level locks

[thomasjiangcy]

Currently, workers use session-level advisory locks. This makes workers incompatible with pgBouncer and requires a direct connection to Postgres.

It would be great if we can move workers to use transaction-level locks because the current implementation doesn't seem to require session-level locks.

[jamescmartinez]

agreed, thanks for the issue! I'll make sure this is included in 1.0.