Add logging for elections

[baumanj]

Currently there is relatively little logging to help understand why an election isn't behaving as expected. This should provide enough detail at the debug level for understanding basic issues and even more at the trace level to help sort out unexpected membership problems.

[thesentinels]

Thanks for the pull request! Here is what will happen next:

1. Your PR will be reviewed by the maintainers
2. If everything looks good, one of them will approve it, and your PR will be merged.

Thank you for contributing!

[raskchanky]

One comment I have in general about debug logging is that tokio is so verbose and frequent with its logging that it has a tendency to drown out everything else at the debug log level.

[christophermaier]

@raskchanky You can tweak the logging levels on a per-module basis, e.g.

RUST_LOG=info,rustc_metadata=error,cargo=error,jobserver=error,rustc_trans=error,rustc_driver=error,rustc_mir=error,rustc=error,tokio_core=info,habitat_sup::manager::service::dir=debug

This helps a lot.

See https://docs.rs/env_logger/0.5.13/env_logger/#enabling-logging for more.

[christophermaier]

A few suggestions / observations, but no blockers.

I really like this... it's like making code comments dynamically available at runtime :)