Use RAII, never plain `lock()`/`unlock()`

Reason

Avoids nasty errors from unreleased locks.

Example, bad

mutex mtx;

void do_stuff()
{
    mtx.lock();
    // ... do stuff ...
    mtx.unlock();
}

Sooner or later, someone will forget the mtx.unlock(), place a return in the ... do stuff ..., throw an exception, or something.

mutex mtx;

void do_stuff()
{
    unique_lock<mutex> lck {mtx};
    // ... do stuff ...
}

Enforcement

Flag calls of member lock() and unlock(). ???