Add read/write locking to QSharedMemory

Locking QSharedMemory can be a heavy operation (on some Qt based Linux devices, it might lock the entire device).

A read/write lock for QSharedMemory would greatly help. For many caches, the reads are very common and writes very rare, so locking would be minimized.

Proposed API extension (based on QReadWriteLock):

QSharedMemory::lockForWrite
QSharedMemory::lockForRead