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.

src/backend/postmaster/autovacuum.c

@@ -529,6 +529,27 @@ AutoVacLauncherMain(int argc, char *argv[])
 	/* must unblock signals before calling rebuild_database_list */
 	PG_SETMASK(&UnBlockSig);
 
+	/*
+	 * Force zero_damaged_pages OFF in the autovac process, even if it is set
+	 * in postgresql.conf.	We don't really want such a dangerous option being
+	 * applied non-interactively.
+	 */
+	SetConfigOption("zero_damaged_pages", "false", PGC_SUSET, PGC_S_OVERRIDE);
+
+	/*
+	 * Force statement_timeout to zero to avoid a timeout setting from
+	 * preventing regular maintenance from being executed.
+	 */
+	SetConfigOption("statement_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
+
+	/*
+	 * Force default_transaction_isolation to READ COMMITTED.  We don't
+	 * want to pay the overhead of serializable mode, nor add any risk
+	 * of causing deadlocks or delaying other transactions.
+	 */
+	SetConfigOption("default_transaction_isolation", "read committed",
+					PGC_SUSET, PGC_S_OVERRIDE);
+
 	/* in emergency mode, just start a worker and go away */
 	if (!AutoVacuumingActive())
 	{
@@ -1530,13 +1551,22 @@ AutoVacWorkerMain(int argc, char *argv[])
 	 */
 	SetConfigOption("statement_timeout", "0", PGC_SUSET, PGC_S_OVERRIDE);
 
+	/*
+	 * Force default_transaction_isolation to READ COMMITTED.  We don't
+	 * want to pay the overhead of serializable mode, nor add any risk
+	 * of causing deadlocks or delaying other transactions.
+	 */
+	SetConfigOption("default_transaction_isolation", "read committed",
+					PGC_SUSET, PGC_S_OVERRIDE);
+
 	/*
 	 * Force synchronous replication off to allow regular maintenance even if
 	 * we are waiting for standbys to connect. This is important to ensure we
 	 * aren't blocked from performing anti-wraparound tasks.
 	 */
 	if (synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)
-		SetConfigOption("synchronous_commit", "local", PGC_SUSET, PGC_S_OVERRIDE);
+		SetConfigOption("synchronous_commit", "local",
+						PGC_SUSET, PGC_S_OVERRIDE);
 
 	/*
 	 * Get the info about the database we're going to work on.