Improve escalus_bosh's handling of session termination

[pzel]

Previously, if an escalus_bosh connection in keepalive mode received a terminate reply from the server, it would send an empty keepalive response back to the server, instead of just stopping.

The server would have destroyed the session by this time, and would print a warning message:

2014-04-15 11:50:14.621 [warning] <0.860.0>@mod_bosh:forward_body:267 session not found!
{xmlel,<<"body">>,
    [{<<"xmlns">>,<<"http://jabber.org/protocol/httpbind">>},
     {<<"type">>,<<"terminate">>},
     {<<"sid">>,<<" {{ SOME_SID }} ">>},
     {<<"rid">>,<<" {{ SOME_RID }} ">>}],
    [{xmlel,<<"presence">>,[{<<"type">>,<<"unavailable">>}],[]}]}

This commit fixes escalus_bosh to stop sending keepalives upon receiving a terminate response.

Additionally, if a connection is timed out on purpose during an escalus story, one may specify it as terminated using escalus_bosh:mark_as_terminated/1. This will prevent escalus_cleaner from sending extraneous session termination requests to the server after the story finishes.

[erszcz]

Merged and tagged as 2.4.1.