[BUG][core] Unguarded access to epolldesc from group sender code

[ethouris]

The problem detected by thread sanitizer: The calls to m_SndEPolldesc->watch_empty() is done without locking, while all operations on epoll require a lock on CEPoll::m_EPollLock.

Changes:

• Made whole CEPollDesc class private with exclusive access for CEPoll. This is to ensure that the CEPollDesc class isn't used "privately" by any other part of the code.
• Added an access function CEPoll::empty to return information about having no subscribers, with appropriate locking.

[maxsharabayko]

Minor suggestion. 👇

[ethouris]

Sorry I had to withdraw this suggestion. This is wrong - although the function is in general read-only, locking the mutex is a writable operation, as per class. Might be that the mutex can be made itself mutable, but this would go too far as a refax and better be postponed.