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

slows down CLUSTER, VACUUM FULL, ALTER TABLE (rewrite & set
tablespace), CREATE INDEX
so basically same things we optimise WAL for and the same places where
we honour maintenance_work_mem
(discuss: should we add COPY, CTAS etc also?)
(discuss: do we need another parameter to specify "cost"? Currently
patch uses "sleep every 64kB of WAL")

VACUUM is not included, since we already have controls for that -
honouring two controls would be complex and weird.