Bluetooth: Mesh: Improve Network Message Cache behavior

The implementation of the Network Message Cache has so far been
suboptimal, since it has treated the same packet with different TTL
values as different packets. Since one of the purposes of this cache
is to prevent unnecessary relaying, it's important that we don't let
the TTL value influence the "hash" that's used for matching messages.

This patch changes the hash to consist of most of the IV Index (three
least significant bytes of it), the sequence number and the source
address, which should give fairly optimal matching behavior.

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