Fix heap profiling regressions.

Remove the prof_tctx_state_destroying transitory state and instead add
the tctx_uid field, so that the tuple <thr_uid, tctx_uid> uniquely
identifies a tctx.  This assures that tctx's are well ordered even when
more than two with the same thr_uid coexist.  A previous attempted fix
based on prof_tctx_state_destroying was only sufficient for protecting
against two coexisting tctx's, but it also introduced a new dumping
race.

These regressions were introduced by
602c8e09 (Implement per thread heap
profiling.) and 764b0002 (Fix a heap
profiling regression.).