* Honor MCLk stop messages, do not auto-start as soon as
the tempo is locked (after 3 clock ticks).
* Retain MIDI beat position, do not rewind in ::update_midi_clock
Do not directly start when receiving the position message,
transport speed should be zero (regardless of clock speed),
until the "continue" message arrives.
* Mclk start needs to rewind (not reset)
There is still an issue with start, when
_session->transport_sample() is not zero. Ardour ends up
constantly locating, rolling for a short time and re-syncing
by locating again.
cf0b119b45