connectionmanager: close all pending callback if all connections fails

If all pending connections fails, it should remove all waiting callbacks
and not only the connecting one.
So, introduce a structure to make the difference between PendingCb linked
to a ongoing DHT request and others waiting for the request to finish.
If all ongoing requests fails, all waiting callbacks are called. This
avoid wrong "Already connecting" messages.

Change-Id: I518f1ef85294f55f78d1b75638b1612c68ea0a0a