n3891 s/shared_mutex/shared_timed_mutex/ is now in C++14

[Ed Smith-Rowland]

Apparently, the class shared_mutex is renamed shared_timed_mutex for C++14.
This shared_mutex was implemented for 4.9 so it hasn't been released as shared_mutex in the wild.
I know it is late for 4.9 but I think we should consider changing this now on trunk so we don't have a released version with something we know is renamed.

I think it is a low risk library-only change. (Famous last words, I know).  No files are renamed.  No guts are changed.  It's purely a name change of the shared_mutex class to be more consistent with the rest of the std library.

Here is a patch.  I'll check it tonight.
I wanted to initiate the discussion now though.

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