trx_toolkit/fake_trx.py: refactor global class hierarchy

This change is a big step towards handling of multiple transceivers
in a single process, i.e. multiple MS and multiple BTS connections.

The old class hierarchy wasn't flexible enough, because initially
fake_trx was designed as a bridge between OsmocomBB and OsmoBTS,
but not as the burst router. There were two separate, but 90%
similar implementations of the CTRL interface, two variations
of each simulation parameter - one for UL, another for DL.

The following new classes are introduced:

  - Transceiver - represents a single transceiver, that can be
    used as for the BTS side, as for the MS side. Each instance
    has its own CTRL, DATA, and (optionally) CLCK interfaces,
    among with basic state variables, such as both RX / TX freq.,
    power state (running or idle) and list of active timeslots.

  - CTRLInterfaceTRX - unified control interface handler for
    common transceiver management commands, such as POWERON,
    RXTUNE, and SETSLOT. Deprecates both CTRLInterface{BB|BTS}.

  - FakeTRX - basically, a child of Transceiver, extended with
    RF path (burst loss, RSSI, TA, ToA) simulation. Implements
    a custom CTRL command handler for CTRLInterfaceTRX.

The following classes were refactored:

  - BurstForwarder - still performs burst forwarding, but now
    it doesn't store any simulation parameters, and doesn't
    know who is BTS, and who is MS. Actually, BurstForwarder
    transforms each L12TRX message into a TRX2L1 message, and
    dispatches it between running transceivers with matching
    RX frequency and matching timeslot.

  - FakePM - still generates random RSSI values, but doesn't
    distinguish between MS and BTS anymore. As soon as a
    measurement request is received, it attempts to find
    at least one running TRX on a given frequency.

Please note that fake_trx.py still does handle only a single pair
of MS and BTS. No regressions have been observed. Both new and
refactored classes were documented.

Change-Id: Ice44e2b22566b3652ef6d43896055963b13ab185
Related: OS#3667

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