Replication info files are not being flushed and synced when the
command 'STOP SLAVE' is issued. This means that one cannot just
rely on existing values on those files when the slave has been
stopped. Having consistent, uncorrupted and up-to-date info files
when stopping the slave would be most useful, for instance, for
snapshotting purposes (a procedure that is often used for
restoring slaves).
      
This patch addresses this by instrumenting the
terminate_slave_threads function so that it also flushes and
syncs the *info files as well as the relay log whenever it gets
called, ie, on 'STOP SLAVE'.  Although this imposes a performance
trade-off (specifically when stopping the slave), it should have
no negative influence on overall replication performance (impact
is only noticeable on 'STOP SLAVE').