Bluetooth: Mesh: Fix adhering to the configured Friend Queue size

Qualification test case MESH/NODE/FRND/FN/BV-08-C requires that we do
not store more messages than the reported Friend Queue size. The
implementation was so far opportunistic and stored more if it could
(it would later discard if necessary to make sure all queues can store
the required amount). The spec also requires the queues to have new
messages overwrite old ones (in the style of a circular buffer), so we
have to keep track of which buffers are part of the same segmented
message (so we discard all buffers belonging to the same message).

To pass the test case, add APIs to check for space in the Friend
queue, and track the number of buffers for each incoming segmented
message.

Fixes #18090

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