Fix initial sync to LTC with small buffersizes

Fix initial sync to LTC with small buffersizes

Ardour follow_slave() does nothing (not even seek) if the slave is not
locked.

The LTC-slave assumes it's locked if LTC is stable for 5 continuous
process-calls.

If the difference of Ardour's transport-position to the LTC-timecode
is large (> 2sec), the slave reset itself (assuming drift, seek don't vari-
speed).

A LTC-slave does reset does reset the locked counter.

Hence: If initially Ardour's transport differs > 2 sec and the buffersize
is small (many process-callbacks), the slave kept resetting itself
never informing Ardour that it locked to the external TC, and Ardour
never issued a seek.