Remove the use of QMutexPool in QHostInfo and QDnsLookup

Q_GLOBAL_STATIC does the thread-safe protection for us. And who said we
could only use non-POD types? We can just use a boolean!

Change-Id: Ifea6e497f11a461db432ffff1449b0a88d75d194
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>