Re: WAL Rate Limiting

We've discussed previously the negative impact of large bulk
operations, especially wrt WAL writes. Patch here allows maintenance
operations to have their WAL generation slowed down as a replication
lag prevention feature.

I believe there was originally intended to be some work on I/O rate
limiting, but that hasn't happened and is in some ways orthogonal to
this patch and we will likely eventually want both.

Single new parameter works very similarly to vacuum_cost_delay

wal_rate_limit_delay = Xms