[core][MAINT] Removed unused and swelling m_pAcceptSockets field + refax

[ethouris]

Fixes #1734

The m_pAcceptSocket field seems to not be used for anything. Sockets were added to it and removed from it under some circumstances, but that's about all.

Refax: Also the m_pQueuedSockets field has been turned into m_QueuedSockets and turned into a solid object instead of pointer. There has never been any reason to make this object dynamically allocated and deallocated and it has only increased entropy.

[maxsharabayko]

I am also voting for making m_QueuedSockets a vector instead of a set.
Only erase operation makes use of a set, but I assume the number of queued sockets is limited by a backlog value, so it is not something where such optimization is beneficial.

[maxsharabayko]

According to the suggestion in comments above the code:

set<SRTSOCKET>::iterator b = ls->m_QueuedSockets.begin();
u = *b;
ls->m_QueuedSockets.erase(b);