core (xmpp): set message timestamp on message reception: message timestamp was set during the parsing, but this could lead to misordering as async calls are done before message is parsed (so a variable delay could happen before the timestamp was set). This patch workaround this by setting timestamp as soon as message is received (in onMessage).

# 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()