The string cache is only very seldom written but very often read. To
improve thread scaling it is faster to lock it only for write operations.
So we use a shared mutex which is locked in shared mode for read
operations and locked exclusively for write operations.

Change-Id: I7dfd4a02c5484683e4d0becd39269c0146126a96
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>