Add externally locked streams

[viboes]

Based on N3354: C++ Stream Mutexes ​http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3354.html add thread safe streams that gets its protecting mutex externally instead of internally as in the proposal so that several streams can share the same protecting mutex.

This will be useful to take in account coherency when working with cout and cin.