MutexProtected mirrors the identically-named Kernel primitive and can be
used to synchronize access to any object that might not be thread safe
on its own. Synchronization is done with a simple mutex, so access to a
MutexProtected object is potentially blocking.
Mutex now has an internal nesting variable which is there to harden it
against lock-unlock ordering issues (e.g. double unlocking).
ConditionVariable is a thin wrapper over the pthread_cond_* APIs, just
as Mutex is a wrapper over pthread_mutex.
Because ConditionVariable might want to wait on a high-level Mutex, it
needs to be friends with it.
