Add graceful shutdown to HTTP server

[hhromic]

While troubleshooting #177, I noticed that graceful shutdown was missing for the HTTP server, making KMinion to hang on interrupt signals, e.g. Ctrl-c, instead of shutting down properly. For example:

# When using Ctrl-c _before_ the HTTP server is started, KMinion shutdowns cleanly
(...)
{"level":"info","ts":"2022-12-13T10:00:49.245Z","logger":"main","msg":"successfully connected to kafka cluster"}
<Ctrl-c>
{"level":"info","ts":"2022-12-13T10:00:49.940Z","logger":"main","msg":"received a signal, going to shut down KMinion"}
{"level":"fatal","ts":"2022-12-13T10:00:50.179Z","logger":"main","msg":"failed to start minion service","error":"failed to check feature compatibility against Kafka: kafka api versions couldn't be fetched: failed to request api versions: context canceled"}

# KMinion exits and returns control to the console

# When using Ctrl-c _after_ the HTTP server is started, KMinion never shutdowns and just hangs
(...)
{"level":"info","ts":"2022-12-13T10:01:13.735Z","logger":"main","msg":"listening on address","listen_address":":8080"}
<Ctrl-c>
{"level":"info","ts":"2022-12-13T10:01:14.410Z","logger":"main","msg":"received a signal, going to shut down KMinion"}

# KMinion never exits from this point and control never returns to the console

This PR fixes the above by properly implementing HTTP server shutdown when the context is cancelled.

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