Disable synchronous commits

[fbac]

By setting SET LOCAL synchronous_commit = off in transactions such as inserting staged envs, originator envs or batches is specially useful because the tx won't wait for WAL to be flushed to disk.

On hard crash the transactions are completely lost and nothing is written, which should be ok given our architecture:

• client/gateway would retry a PublishClientEnvelope if inserting a staged envelope fails.
• xmtpd publish worker would retry publishing a staged envelope
• migrator retries publishing a batch

[tylerhawkes]

This should be the last optimization that we try. Having all the database guarantees in place is always nice to have if possible.

[fbac]

Completely agree. This wouold be the last resort, and most probably we don't need this anyway.