The periodic timers fire when the current time exceeds the
start time + the period. However, without reseting the start
time, the timer would end up thinking it must fire immediatly.

By reseting the start time when firing, the timer will
only fire again when another period has expired, not immediatly.

Change-Id: Ibd0311b12a514bfd558c0bd6ef83df8c89fd8c7e
Reviewed-on: https://gerrit.libreoffice.org/17194Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>