pgsql: Reduce the default value of autovacuum_vacuum_cost_delay to 2ms.

Reduce the default value of autovacuum_vacuum_cost_delay to 2ms.

This is a better way to implement the desired change of increasing
autovacuum's default resource consumption.

Discussion: /messages/by-id/28720.1552101086@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cbccac371c79d96c44fcd8c9cbb5ff4dedaaa522

Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
src/backend/utils/misc/guc.c | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)