This patch adds for "--ps-protocol" second execution
of queries "SELECT".
Also in this patch it is added ability to disable/enable
(--disable_ps2_protocol/--enable_ps2_protocol) second
execution for "--ps-prototocol" in testcases.
Consistent with MDEV-4206 and empty log_slow_filter still means
no explict filtering. Since 21518ab2e4 however the
log_queries_not_using_indexes became stored in the same variable.
As we need to test for the absense of log_queries_not_using_indexes
the SERVER_QUERY_NO_INDEX USED part of log_slow_statement, the empty
criteria resulted in an always true to log queries not using indexes if
log_slow_filter was set to empty.
Adjusted the log_slow.test for MDEV-4206 as slow_log_query has been
global and session for a while and it was relying on the MDEV-21187
buggy behavior to detect a slow query.
Reviewer: Monty
