unexpected ack message
We sometime have this message:
2017-04-06 14:40:17 =ERROR REPORT====
Supervisor received unexpected message: {ack,<0.829.0>,{ok,<0.829.0>}}
It might be send by barrel_httc which has lost its connection (due to cowboy timeout) and retry.
The proc_lib:init_ack
should probably be done only once at process start, and not at each connection retry.