Logging fixes: 501 should be a warning, don't log 4XXs

[amwolff]

Goal

4XXs are a normal operation and are cluttering the INFO level.

This is also the case with 501s ("Not Implemented") on the ERROR level.

Acceptance Criteria

### Tasks
- [ ] Move 4XXs to DEBUG
- [ ] Move 501s to WARNING

[storj-gerrit[bot]]

Change pkg/httplog: change log levels for 4xx and 501 status mentions this issue.

[storj-gerrit[bot]]

Change pkg/server/middleware: add additional fields to gmt events mentions this issue.

[storj-gerrit[bot]]

Change {pkg/httplog,pkg/server/middleware}: move common log code to httplog mentions this issue.