also move the MutexGuard in revokeClient. No need to hold it while doing
an osl_atomic_increment. Save it until we actually need to call
onLastClient.

Change-Id: Ia87f24b66b0430604a2fe717c95223ff1f5ece6d
Reviewed-on: https://gerrit.libreoffice.org/20919Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>