Previously, the Lock was not thread-safe when multiple threads were
locking/unlocking a single Lock instance simultaneously. This
isn't normally a problem when using the MessageManagerLock type, because
each MessageManagerLock contains its own private MessageManager::Lock,
and it's not possible for multiple threads to modify that private lock.

This change improves safety when using a MessageManager::Lock in the
manner of a CriticalSection or other JUCE lock type.