rtcp: add thread safety for prticipants_ data structure

[wowaser]

fix for #172

[jrsnen]

Hi and thanks! This is close to what is needed.

I looked up and shared_mutex was only introduce in C++17 which might cause problems for some of our users who use older versions of C++ standard (if I remember correctly, few have been using C++14). Would plain std::mutex be good enough?

Also, while most of it comes down to personal preference, I like explicitly locking and unlocking the mutexes using lock/unlock functions whenever possible just to it is easier to see which areas the mutex locks (the goal always being to lock as little code as possible to avoid delays). Could you at least replace the blocks created for locking the mutex so the code becomes clearer?

[wowaser]

@jrsnen Sure, I will still use unique_lock(since c++11) as locking mechanism to avoid errors in the future

[wowaser]

@jrsnen Take a look please
I used classic mutex locking/unlocking and lock guards. In a couple places I also used unique_locks for the purpose of unlocking praticipants_ if there is an error or premature return.

[jrsnen]

@wowaser this is great. Big thanks for this PR! Sorry, it took me so long to review it.