Remnants from the olden days of the big OpenZAP migration.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

FS-4525 --resolve ok there was 2 parts to this and I missed one, your client actually uses different to and from in the subscribe where the majority use the target name in the request, to and from, this should make it compat

Move the B-channel message handling into a per-span I/O thread,
to solve most of the problems caused by the intermixed data + control
socket interface of mISDN, missing write poll() support on
mISDN B-channels and the FreeTDM I/O model. This eliminates most of
the audio problems (except for a few minor glitches).

A unix stream socket pair is used as a bi-directional pipe replacement
(the pipe code is still included in this commit, but will be removed later),
with the RX and TX buffer sizes carefully tuned to avoid excessive buffering
(= latency) and a deadlock situation between the write() call in ftdm_write()
and the code in misdn_span_run() that needs a minimum amount of data in the
TX buffer, before sending out a PH_DATA_REQ to the mISDN socket
(see misdn_span_run() comments for more details).

The minimum size for pipes is PAGE_SIZE (4k), which is ~500 ms worth of
audio. A socket pair RX/TX buffer size of 3k, seems to hold a maximum
amount of around 500 bytes data in practice, giving us a much lower
maximum latency than a unix pipe. (The socket pair might be replaced by a
custom ring buffer / fifo data structure to get even more fine grained
control of the maximum latency.)

The newly introduced span_start / span_stop callbacks in
ftdm_io_interface_t are used to start / stop the I/O thread. The callback
functions will wait up to 10 seconds for the thread to successfully
start up or shut down (using a mutex + condition var).

NOTE: Using any of the locking ftdm_span_() functions in the I/O will cause
      a deadlock between the I/O thread (trying to lock span->mutex) and the
      thread calling ftdm_start()/_stop() (holding the span->mutex).
      (The I/O thread currently uses direct span member access to avoid this.)

The I/O thread uses the epoll(7) family of functions for event handling.
An epoll context is created on startup and all B-channel sockets are
registered (READ, PRI and ERR). Before entering the event loop,
the I/O thread will send a signal on the condition variable, to
indicate it has completed the startup procedure.

Incoming b-channel and command pipe events are handled by the event loop.
Payload of incoming PH_DATA_IND frames (= audio data) is sent to the
rx_audio_pipe_in end of the b-channel's socket pair and, if enough data is
available, a PH_DATA_REQ of the same size is sent to the b-channel mISDN socket
to transmit audio.

A MISDN_CMD_STOP command on the event pipe will wake up the I/O thread and
cause it to shut down. All b-channels will be unregistered from the epoll context
and the epoll fd closed. The I/O thread terminates itself after signalling the
successfull shutdown on the condition variable.

TODOs:
    - Move D-Channel into I/O thread too

    - Custom FIFO/ring buffer for data (even lower latency)

    - Improve epoll() code (per-channel struct w/ callback, for epfd.data.ptr)

    - Use mISDN DSP for audio (e.g. tone generator, dtmf detector, echo cancel)

    - Use a per-port / span control socket to execute channel commands
      synchronously, or add misdn_commands (queue?) that can be used that way

    - Name I/O threads 'mISDN-%SPAN_NAME%', e.g. 'mISDN-M_BRI1'
      (= add ftdm_thread_set_namef(thread, fmt, ...) / ftdm_thread_set_name(thread, name))

TL;DR: "tweak", solves "booboo" with audio
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

Fixes:
    cc1: warnings being treated as errors
    src/switch_core_session.c: In function 'switch_core_session_thread_pool_worker':
    src/switch_core_session.c:1478: warning: format '%ld' expects type 'long int', but argument 9 has type 'switch_size_t'
    src/switch_core_session.c:1483: warning: format '%ld' expects type 'long int', but argument 9 has type 'switch_size_t'
    gmake[2]: *** [libfreeswitch_la-switch_core_session.lo] Error 1
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>