osmux: Set M bit in osmuxhdr if seqnum hole found encoding RTP pkts

So far only small intra-batch seqnum jumps are filled in with forged RTP
packets when storing the incoming RTP packets.
Under some conditions, holes may still exist in the queue of RTP packets
for a stream:
* Seqnum detected when first incoming RTP in batch is queued (this can
  be improved in the future).
* Big seqnum jumps > batch_factor or simply filling out of bounds for currently
  enqueued batch.

Specially the second case can come from long network dropouts, or simply
due to a bug in the RTP being feed to osmux layer (be it from local code
or peer). In that case (long jumps) we don't want to generate tons of
packets filling in several entire batches (potentially incredibly big
amount of batches).
Instead, in these scenarios, simply let the osmux peer know there was a
jump by setting the M bit on the next osmux header for that circuit
after the seq jump has been detected.

Related: SYS#6161
Change-Id: I05b1eae400cb60d1f4e927f853619d5ff470163f
---
M src/osmux_input.c
M tests/osmux/osmux_input_test.c
M tests/osmux/osmux_input_test.ok
M tests/osmux/osmux_test.c
M tests/osmux/osmux_test.ok
5 files changed, 155 insertions(+), 141 deletions(-)