Bluetooth: Mesh: Create dedicated helper for incrementing seq

The sequence number is incremented from several different places in
the stack. The way it was done was potentially race condition prone,
and was also problematic from the perspective of updating the
sequence number in persistent storage. Create a dedicated helper for
incrementing the sequence number (solves the race) which can in later
patches be used to add the persistent storage support.

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