Lightweight Mutex

[eruffaldi]

CoCo and SRGViz use extensively mutexes and in particular we employ boost::shared_mutex (now in C++17) for supporting in particular lock upgrades. Unfortunately these mutexes are quite heavyweight as shown from tests online (https://www.arangodb.com/2015/02/comparing-atomic-mutex-rwlocks/).

We should evaluate to introduce the use of RWLocks, available in pthread and also in modern Windows, to speed-up lock acquisition in core CoCo/SRGViz structures.

1. Find spots for improvements
2. Pick library providing lightweight mutexes
3. Replace!
4. Profile