sr: session: Deprecate sr_session_halt()

sr_session_halt() in its current state is kind of useless and even dangerous.
All it will do is mark the session as not running, but wont signal the devices
to stop to capture data. This is not so much of a problem with the blocking
sr_session_run(), but once there is support for asynchronous data acquisition by
attaching the session sources to the applications mainloop sr_session_halt()
basically becomes a no-op. sr_session_stop() already does what needs to be done,
marking the session as not running and signal the devices to stop acquisition,
so make sr_session_halt() an alias for sr_session_stop() and deprecate its
usage.

Signed-off-by: Lars-Peter Clausen <redacted>