Improve escalus_bosh's handling of session termination

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:
``` erlang
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.