osmux: Support recreating lost RTP packets at start of the batch

Previously, if RTP jumps were detected in the incoming RTP stream and
osmux state for that circuit was to start the next batch, the hole would
not been filled during queueing time and instead the encoder would have
set the M bit in the osmuxhdr to announce a sync point.
For small holes (eg less than the batch factor) it makes sense to start
filling the batch with crafted RTP packets in order to avoid the encoder
later on setting the M bit and hence avoid the peer receiving the Osmux
frame having to start a new syncrhonization point.

Related: SYS#6161
Change-Id: I9596501adf5b7b91983618c92c7b1792ee9461a3
---
M src/osmux_input.c
M tests/osmux/osmux_test.c
M tests/osmux/osmux_test.ok
3 files changed, 626 insertions(+), 426 deletions(-)