smartagent: refactor logrus shim and support monitorID field

[rmfitzpatrick]

These changes adopt the monitorID field inclusion provided by signalfx/signalfx-agent#2334 as well as refactor the existing logrusToZap logic:

1. Relocate all unredirect logic from the logrus shim to the tests since we should never stop listening to a monitor's logger just because the parent receiver has been stopped. Afaict this was originally added for the tests to pass.
2. Remove redundant default zap logger creation since we should always be using the actual service logger. Rewriting the name field with default is my proposed change.
3. General var naming and comments for clarity since the current functionality is a bit cryptic imo.*
4. Improve some test practices, for var reference bungs, and handle missing fatal/panic level cases.