Bluetooth: Mesh: Fix accuracy of Publish Period

After the Publish Retransmit state was introduced the Publish Period
measurement would begin once the previous Publish message has finished
transmitting. This will however cause inaccurate periods, which is
particularly an issue with the PTS that expects accuracy of less than
0.5 seconds (apparently).

Since the publication timer is also used for the retransmissions we
can't simultaneously use if for the period as well. Therefore, we
introduce a new variable called period_start which makes a note of
when the period was supposed to start, and then once all
retransmissoins are done initializes the timer with the send duration
taken into account.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>