model: refactor mutex thread-blocking code

model: refactor mutex thread-blocking code

There are a few functions that are used in suboptimal ways.

* use the ModelChecker::get_thread(ModelAction *) version of overloaded
  'get_thread()' function

* the add/remove interfaces were used inconsistently previously; for
  "waking" we directly utilized Scheduler::add_thread, whereas the
  "sleeping" case used ModelChecker::remove_thread

* use the Scheduler::sleep and Scheduler::wake functions for sleep/wake
  instead of explicitly adding/removing