jamiaccount: do not cancel connecting request on sip timeout

During a connectivityChange(), a SIP channel can be used to send
messages but will close a few seconds after (due to a timeout or keep
alive). However, every message failing was closing connections
(including the current outgoing requests) and re-asking a socket.
Causing multiple connection request to be sent (because the request
was erased just after).
Also, to avoid to ask for multiple SIP channels on the same socket
this patch introduces ConnectionManager::isConnecting().

Note: erasing the request was causing a lot of "no response from DHT"
in the logs because the request was destroyed directly.

Change-Id: Ie06f377743075d4a6cb6a4bb6c940bd7b23f1603
GitLab: #512
GitLab: #421