Use the "==" operator instead of pthread_equal().

This allows us to eliminate two boolean flags which are there to avoid
race conditions, and made the thing so fragile that I got tons of
(correct) thread sanitizer warnings.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Since we switched to C++ exceptions, there is no code path which
returns false.

The class does not have any virtual methods, and thus the (debug-only)
destructor does not need to be virtual.

Remove a GLib dependencies from class EventLoop and DatabaseLock.