Teach node_ctl, node_pause and node_run immediate 'now' mode
Add a default ached ahead time
Optimise counter:

There's no need to use heavyweight atom abstraction, a simple mutex
will suffice. Should reduce contention and blocking between music threads.
Teach server thread about :sonic_pi_thread_group and priority
Make thread priorities more diverse for a greater effect
Fix bug in doc name for with_sample_pack
Further thread prioritisation changes
Use default_sched_ahead_time rather than a magic number
Further doc additions