rtp: Hide RTP implementation details from module-rtp-*

rtp: Hide RTP implementation details from module-rtp-*

This moves RTP implementation-specific information out of
module-rtp-send/recv. This is basically done by making the
pa_rtp_context structure opaque from the perspective of these modules.
We can then potentially replace the underlying RTP implementation with
something else transparently.

One RTP detail that does "leak" is the RTP timestamp. We provide this to
module-rtp-recv so that it can perform rate adjustments to match the
sender rate.

Signed-off-by: Arun Raghavan <arun@arunraghavan.net>