DRTVWR-476: Make LLVivoxVoiceClient::logoutOfVivox() wait for logout.

It can happen that we arrive at logoutOfVivox() with some other message
queued on the LLEventMailDrop in question. If logoutOfVivox() assumes that
other message is logout and exits, then subsequent code gets confused.
Introduce a loop to wait (with the existing timeout) for the real logout
message.