pgsql: Reduce the default value of autovacuum_vacuum_cost_delay to2ms.

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: https://postgr.es/m/28720.1552101086@sss.pgh.pa.us

Branch
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master

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

Modified Files
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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(-)