[Stop using ATL] Convert ATL mutexes to standard library ones.

[eric@adblockplus.org]

Background

The current code uses the ATL class CComAutoCriticalSection. The C++11 standard library provides std::mutex for this purpose.

What to change

Replace CComAutoCriticalSection with std::mutex. As a rule, locking a mutex should be done by std::unique_lock for exception safety.

[sergz]

I would recommend to use std::recursive_mutex instead of std::mutex, it's much safer. And std::lock_guard instead of std::unique_lock when it's possible, because in most cases we need the functionality of only lock_guard, so we don't expect anything more, the result is less number of mistakes and better readability. Not sure, but lock_guard can be even lighter.