Bluetooth: Mesh: Check for duplicates in friend queue

Mesh Profile Specification Errata 11302

Section 3.5.5.

When a Friend node receives a message that is destined for a Low Power
node (i.e., the destination of the message is a unicast address of an
element of the Low Power node or in the Friend Subscription List), and
the TTL field has a value of 2 or greater, then the message shall be
processed as follows: If the Friend Queue already contains a message
with the same SEQ and SRC fields as in the received message, or if the
SRC field of the received message is a unicast address of an element of
the Low Power node, then the message shall not be stored in the Friend
Queue. Otherwise, the TTL field value shall be decremented by 1, and
the message shall be stored in the Friend Queue.

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>