pg_xlog_wait_remote_apply (and _receive) don't check for cancel requests

https://support.2ndquadrant.com/rt/Ticket/Display.html?id=7203
Lower log levels of some common messages

https://support.2ndquadrant.com/rt/Ticket/Display.html?id=7202
Lower log level for bdr_apply and make it configurable

Per discussion on https://rt.2ndquadrant.com/rt/Ticket/Display.html?id=7202