core (xmpp): avoid starting several clients at the same time: a check is done to avoid running startConnection several times at once, which would lead to the creation of several SatXMPPEntity instances at the same time, resulting in many issues. If startConnection is called while a previous one is not finished yet, a CancelError is now raised.

# XXX: the Monkey Patch is here and not in src/__init__ to avoir issues with pyjamas compilation
# TODO: remove this when changes are merged in Wokkel
from sat_tmp.wokkel import install
install()