Decoupled wsrep thread count from connection count. By doing so,
the number of wsrep threads (applier/rollbacker) would no longer
affect the threads_connected status variable and thus maximum
allowable user connections limit would be @@max_connections.
Also introduced a new status variable 'wsrep_thread_count' to hold
the number of wsrep applier/rollbacker threads.
Added a test case.
mysql_tzinfo_to_sql tries to set wsrep_replicate_myisam
as SESSION variable, while its a GLOBAL-only system variable,
and thus the resulting sql would fail to load.
Fixed by changing the scope to GLOBAL in the SET command.
Also added following include files to facilitate testing :
* include/have_wsrep.inc
* include/not_wsrep.inc
* include/have_wsrep_enabled.inc
