Remove a GLib dependencies from class EventLoop and DatabaseLock.

Use CRITICAL_SECTION and CONDITION_VARIABLE.  This requires Windows
Vista or newer.  It fixes problems with GLib threading objects that
were implicitly created by static constructors before g_thread_init().

Switch WIN32 to this implementation to be able to use condition
variables, which is impossible with CriticalSection.

Use a custom pthread_mutex_t wrapper because std::mutex adds overhead.