The combination of only one slave being used and the slave_selection_criteria
being LEAST_CURRENT_OPERATIONS can possibly cause a connection pileup on one
slave server. This would skew the query distribution heavily towards the first
available slave even if multiple slave were being used.
Having the maximum number of slave servers to be equal to the total amount of
available slaves allows for a more even and responsive distribution of the
query traffic.
Before these changes when max_sescmd_history was used the session
was closed when the limit was exceeded. With this change, when the
limit is exceeded the recovery of slaves and the session command history
are both disabled. This will allow the sessions to continue while still
keeping the old functionality of limited salve replacement.
The disable_sescmd_history and disable_slave_recovery parameters were combined
so that disabling the session command history will also disable slave recovery.
This way no harm can be done with disable_sescmd_history.
