connectionmanager: avoid multiple request if multiple connectDevice

Before this patch, connectDevice() triggered a new ICE negotiation
until no TLS socket exists. This means, if when starting a
communication with a Device by triggering two connectDevice() like
a SIP channel and a data-transfer, two requests will be send.
Now, connectDevice will store new connectDevice() requests into
pendingCbs to only do one request at a time.
Note that the other side can ask a new request via the DHT, in this
case, behavior will not change, we accept all requests if
authorized and this must not interact with the pendingCbs_

Change-Id: I42a12458c364174e08531f568826da5b8348e08a