memberlist.Logger Interface, More Flexible Logger Replacement

[michaelquigley]

Implemented a memberlist.Logger interface, allowing for more flexible logger library replacement (we use logrus). Also removes a bunch of [LEVEL] memberlist: boilerplate from log messages.

Care was taken to handle all of the logging specializations in the test cases.

[michaelquigley]

Not sure what the failing test is, but locally I ran into a number of tests that don't seem to execute reliably.

If there's a go fmt issue, please let me know and I'll fix it promptly.

[hashicorp-cla]

All committers have signed the CLA.

[michaelquigley]

CLA signed.