feature: Disable load balance after a SELECT having functions specified in black...

feature: Disable load balance after a SELECT having functions specified in black/white function list.

In Pgpool-II 4.0 or earlier, if we set "disable_load_balance_on_write = transaction",
when a write query is issued inside an explicit truncation,
subsequent queries should be sent to primary only until the
end of this transactionin in order to avoid the replication
delay.

However, the SELECTs having write functions specified in black_function_list
are not regarded as a write query and the subsequent read queries are still load balanced.

This commit will disable load balance after a SELECT having functions
specified in black/white function list.