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.
