Changes

CSD: implement half-rate modes correctly

TCH/H4.8 and TCH/H2.4 modes are different from all other TCH in that
the channel coder runs (statistically) every 40 ms instead of the usual
20 ms.  However, the standard RTP interface of TS 48.103 still requires
20 ms packets for all CSD modes; furtherfore, this requirement is not
just a matter of 3GPP spec being obstinent, but is highly useful for
interoperability, both between FR and HR and between OsmoBTS and E1 BTS.

The handling of CSD HR was totally broken in this regard: wrong RA2
packing mode, RTP clock model was violated, and even an internal
mismatch with l1sap sending TCH.req prims at twice the rate
at which the PHY expects them.

With this patch CSD HR handling becomes correct for TCH/H2.4 and for
TCH/H4.8 transparent; fully correct handling for TCH/H4.8 NT will
appear in a follow-up patch.

The packet/frame rate mismatch (40 ms Rx/Tx times while each RTP packet
carries 20 ms worth of data) is reconciled by emitting two RTP packets
directly back-to-back for UL, and pulling two RTP packets at a time
from the Rx jitter buffer at the needed times for DL.

Note: due to bug OS#6604, TTCN3 tests for CSD half-rate transparent modes
will start failing once this patch is merged; to make them pass again,
TTCN3 test code will need to be reworked to fix that bug.

Related: OS#6577
Change-Id: Ib35e910df263743cd243f51fb0bd6551ddfcf4c5