Replace the multiple custom assert wrappers with a single set

Previously, different files had different sets of wrappers around
pthread_mutex_* and rwlock_* functions.  This commit removes those
and puts assert_ prefixed ones in threadwrap.h and rwlockwrap.h.

It also uses them in all the bits that SyncTERM uses.

This means debug builds are much more likely to unexpectedly
terminate with an assertion... when it does, let us know!