mxsock: remove eventLoop from destructor

ConnectionManager MUST be the one managing multiplexed sockets.
However, ChannelSocket are passed as shared_ptr, so the destruction
can happen anywhere. So, we need to avoid to wait for the
eventloop in the destructor, but, let the ConnectionManager do this.

Also, on a related, sendBeacon should be used when sipConnsMtx_ is
not locked, as it's not related and was causing a deadlock if
the multiplexed socket was destroyed in this function.

GitLab: #602
Change-Id: I09449ff727d7003da9f815aff8dd9c78dae53a4e