Add termination hint in bosh timeout test, start metrics helper w/checks

Escalus >= 2.4.1 provides the low-level function `escalus_bosh:mark_as_terminated/1`, which lets the programmer mark a client connection as not eligible for closing.
This is used in the `disconnect_inactive` test to silence 'session not found!' warnings on the server, triggered by `escalus_cleaner` trying to close the session.

OTP 17 is included in `rebar.config`

`metrics_helper` makes sure that all the applications it depends on are launched before starting `lhttpc`.