Transceiver52M: implement TRXDv2 support

TRXDv2 was proposed and implemented in osmo-bts a while ago, but
osmo-trx was still stuck at TRXDv1.  This patch implements TRXDv2
and UL/DL burst batching using libosmo-trx API.

DL: driveTxPriorityQueue() now loops over osmo_trxd_burst_req_parse()
to handle datagrams potentially containing multiple batched BURST.req,
dispatching each via the new handleBurstReq().

UL: accumulate BURST.ind PDUs for the same FN into one msgb per channel
and flush as a single datagram once the next frame's PDU arrives or
the channel stops;  TRXDv0/v1 channels keep sending one PDU per
datagram via sendBurstInd().  Batching is unconditional for now;
making it configurable is left as a follow-up.

Change-Id: Ie6a33e3980b3ca4ab05a3d4221d5a8ba0132233d
Related: OS#5283
---
M Transceiver52M/Transceiver.cpp
M Transceiver52M/Transceiver.h
2 files changed, 173 insertions(+), 56 deletions(-)