This patch fixes two problems that may arise when changing the
value of wsrep_slave_threads:

1) Threads may be leaked if wsrep_slave_threads is changed
   repeatedly. Specifically, when changing the number of slave
   threads, we keep track of wsrep_slave_count_change, the
   number of slaves to start / stop. The problem arises when
   wsrep_slave_count_change is updated before slaves had a
   chance to exit (threads may take some time to exit, as they
   exit only after commiting one more replication event).
   The fix is to update wsrep_slave_count_change such that it
   reflects the number of threads that already exited or are
   scheduled to exit.

2) Attempting to set out of range value for wsrep_slave_threads
   (below 1 / above 512) results in wsrep_slave_count_change to
   be computed based on the out of range value, even though a
   warning is generated and wsrep_slave_threads is set to a
   truncated value. wsrep_slave_count_change is computed in
   wsrep_slave_threads_check(), which is called before mysql
   checks for valid range. Fix is to update wsrep_count_change
   whenever wsrep_slave_threads is updated with a valid value.