HPCC-32960 Fix epoll memory corruption issues

Although the SelectItem 'items' array is protected by a CS,
epoll_wait can receive pending events for SelectItems that have
been deleted, which were then link counted and caused corruption.

Like the select handler version, avoid deleting the SelectItem's
when items are removed, instead mark them deleted and let the
triggerselect() approach remove them on the main thread.

Also fix a theoretical issue when adding a existing socket to
CSocketEpollThread, it did not check other CSocketEpollThread's
to see if it needed to be removed. (given default hdlPerThrd is
UINT_MAX, it will not have happened).

Signed-off-by: Jake Smith <jake.smith@lexisnexisrisk.com>