Increase the default vacuum_cost_limit from 200 to 2000

Increase the default vacuum_cost_limit from 200 to 2000

The original 200 default value was set back in f425b605f4e when the cost
delay settings were first added.  Hardware has improved quite a bit since
then and we've also made improvements such as sorting buffers during
checkpoints (9cd00c457e6) which should result in less random writes.

This low default value was reportedly causing problems with badly
configured servers and in the absence of a native method to remove
excessive bloat from tables without incurring an AccessExclusiveLock, this
often made cleaning up the damage caused by badly configured auto-vacuums
difficult.

It seems more likely that someone will notice that auto-vacuum is running
too quickly than too slowly, so let's go all out and multiple the default
value for the setting by 10.  With the default vacuum_cost_page_dirty and
autovacuum_vacuum_cost_delay (assuming a page size of 8192 bytes), this
allows autovacuum a theoretical maximum dirty write rate of around 39MB/s
instead of just 3.9MB/s.

Author: David Rowley

Discussion: https://postgr.es/m/CAKJS1f_YbXC2qTMPyCbmsPiKvZYwpuQNQMohiRXLj1r=8_rYvw@mail.gmail.com