XEP-0313 MAM support.

[thibaultmarin]

This is an attempt at implementing support for XEP-0313 Message Archive Management (http://xmpp.org/extensions/xep-0313.html).

I am testing this with a prosody server (Prosody trunk nightly build 732 (2016-12-19, 8efd51667622)) and mercurial prosody-modules (https://hg.prosody.im/prosody-modules/ changeset 2462:81127dcdb326).

It would be useful if people could test this and report.

[thibaultmarin]

Hi, thanks for the review.

DamienCassou commented on this pull request. +(defcustom jabber-history-mam nil

shouldn't that be a per-account option instead?

The current scope follows that of the `jabber-history-enabled' option. Would it make sense to keep both options with the same scope in this pull request, and possibly make the options account-specific at a later time? I'd rather isolate the MAM implementation from other improvements, if that makes sense.

That being said, the code would hang when MAM is enabled but not supported by a server, so I have implemented a tentative fix for that.

Functions in these files are quite long. Your code would gain readability if some smaller functions were extracted from the large ones.

I updated the code, splitting functions into smaller components. I hope it improves readability.