Changes

[WIP] Revert "Revert "trx_toolkit/transceiver.py: implement the transmit burst queue""

This reverts commit d4ed09df57b3461470af501e9687ddd80eb78838,
reinstating tx queue into fake_trx.

It is ok to do so because, as explained in abc63d8d (trx_toolkit/clck_gen.py:
Fix clock generator not to accumulate timing error), the
reason for GSM clock jitter problem was timing error accumulation in
CLCKgen, not problems with py threading.Event.

Note: this restores original tx queue implementation basically as-is
with only resolve minor conflicts during the revert. The original tx
queue implementation wastes CPU cycles though because it linearly scans
the whole tx queue at every TDMA frame. If that CPU usage becomes a real
problem it should be straightforward to fix by reworking tx queue to use
priority queue instead of unordered array via heapq module from standard
library. See https://docs.python.org/3/library/heapq.html for details.

(WIP because I did not tested this patch fully as I could not yet
organize a working fake_trx setup due to not fully done homework yet)

Related: OS#4658, OS#6672
Change-Id: I41291708effdd2c767be680fff22ffbd9a56815e