model: proactively build/prune is_enabled() set

model: proactively build/prune is_enabled() set

We should proactively check whether threads are enabled, as this allows
the model-checker to more accurately reason about backtracking, fairness
(particularly, assigning thread priorities), and sleep sets.

This commit causes changes in observed executions for many test
programs. Often, this occurs for programs where the main thread
performs a JOIN on a child thread. Previously, we would only discover
that this thread should be disabled once the scheduler attempts to
execute the JOIN ModelAction. For the period of the trace in which this
disabling isn't discovered, we may try to set backtracking points for
the main thread. These executions just turn into infeasible or
sleep-set-redundant executions anyway.

This can also affect the assertion of thread priority when assigning
backtracking points. If a thread is supposedly "enabled" (but in fact is
not), then we may prevent other threads from running because this thread has
priority. But that thread cannot run, so instead we were doing some
funky things with the remaining ("low-priority") threads.

This fix remedies these sorts of issues by precisely computing the set
of threads which are disabled, waiting on other threads/locks.