Amp repositioning

[pzel]

XEP-0079 specifies the following work-flow for processing AMP-tagged messages:

2.2 Server Processing
Server operation is where the bulk of the work is performed. Upon receiving a message with an AMP
extension, the server performs the following flow:

• Validate the semantics (E1, E2).
  • Determine the default behavior.
• Process rules until condition is met.
• If a condition is met, execute that rule's action
  If no conditions are met, perform "default" behavior for message
• Execute determined action (UCE) (E3).

This PR moves AMP strategy resolution from the filter_packet hook to a new hook in ejabberd_c2s.
This hook (amp_check_packet) is run in the transistion between the session_established and session_established2 states. Checking AMP this early in the pipeline makes sure that no server-side effects (like user_send_packet) are triggered before the AMP strategy is decided.

Also, because http://xmpp.org/extensions/xep-0079.html#security is not implemented, mod_amp is disabled in the default ejabberd.cfg.

[mongoose-im]

travis is using test branch amp-repositioning from https://github.com/esl/ejabberd_tests/tree/amp-repositioning

[pzel]

ejabberd_tests has a separate branch for this pull request, but there are no changes from master.

[michalwski]

I'm deleting this branch (from ejabberd_tests) as it is not up-to-date with master and sic_SUITE always fails.