Reduce log noise of metrics submission

[satta]

FEVER currently logs something like

{"domain":"submitter","level":"info","msg":"submission to <redacted> successful","payloadsize":117,"rawsize":117,"submitter":"AMQP","time":"2021-05-04T07:34:59Z","url":"..."}

whenever a submission to an AMQP sink is completed. In the majority of cases, this leads to massive log noise which does not add much information as errors are logged as well with different log level. These messages should be moved to log level debug instead of info.

[satta]

Closed via #80