plugin XEP-0045: join / MAM history improvements: - fixed use of history_d deferred so MAM history is stored in database as soon as possible and in right order - get MAM history with pages of 20 messages instead of 100 - if a message can't be parsed, an error message is logged (with a dump of the stanza), and the message is ignored, avoiding a crash and the impossibility the join the room at all - a new `mucRoomPrepareJoin` signal, containing room jid and profile, is sent when a room joining is started, this allow better UX as the frontend can show the room immediately, and lock it with a waiting message until it is fully available. The `mucRoomJoined` is still sent when the room is fully joined/avaiable, meaning that the frontend can unlock it and let the user interact with it

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ping: get response time of an XMPP entity
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``ping`` is more or less the equivalent of the command of the same name on most OSes, but
for XMPP. It send a `XEP-0199`_ PING and wait for the answer. When (and if) received, the
time to receive the answer is shown, else the time to receive the error message is shown.
This can be helpful to quickly test the connection with the server or a device.

If you need to get only the response time (without text around), you may use ``-d,
--delay-only``.

.. _XEP-0199: https://xmpp.org/extensions/xep-0199.html

example
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Get reponse time of a server::

  $ jp ping example.org