pgsql: Prevent autovacuum transactions from running in serializable mod

Prevent autovacuum transactions from running in serializable mode.

Force the transaction isolation level to READ COMMITTED in autovacuum
worker and launcher processes. There is no benefit to using a higher
isolation level, and doing so could result in delaying foreground
transactions (or maybe even causing unnecessary serialization failures?).
Noted by Dan Ports.

Also, make sure we disable zero_damaged_pages and statement_timeout in
the autovac launcher, not only workers. Now that the launcher can run
transactions, these settings could affect its behavior, and it seems
like the same arguments apply to the launcher as the workers.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/73d1bfd0b567ef9a4439be1510c12c346a064d9e

Modified Files
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src/backend/postmaster/autovacuum.c | 32 +++++++++++++++++++++++++++++++-
1 files changed, 31 insertions(+), 1 deletions(-)