Add a testable_mutex

[viboes]

Based on Associate Mutexes with Data to Prevent Races, By Herb Sutter, May 13, 2010 - ​http://www.drdobbs.com/windows/associate-mutexes-with-data-to-prevent-r/224701827?pgno=3

"Make our mutex testable if it isn't already.

Many mutex services (including boost::mutex) don't provide a way to ask, "Do I already hold a lock on this mutex?"

Sometimes it is needed to know if a method like is_held to be available.

This wrapper associates an arbitrary lockable type with a thread id that stores the ID of the thread that currently holds the lockable. The thread id initially holds an invalid value that means no threads own the mutex.

When we acquire a lock, we set the thread id; and when we release a lock, we reset it back to its default no id state."

[viboes]

Committed in trunk revision [81088].

[viboes]

Committed revision [81667].