[core] Replaced CGuard with corresponding lock: UniqueLock or ScopedLock

[maxsharabayko]

Addresses an item in #1103.
CGuard was used as a temporal solution to avoid too many merge conflicts with the bonding functionality.
Now most of that functionality is merged, so it is relatively safe to replace CGuard with the corresponding lock: ScopedLock or UniqueLock.

[ethouris]

Some minor things to fix in the comment descriptions. For the rest of the cases I state that UniqueLock was used exclusively in situation when there's needed the use of CV or enforced or scoped lifting of the mutex and ScopedLock everywhere else.