Don't rewrite volatile sentinel config file

[joshbenner]

This PR tries to address the issue of Sentinel (and redis-server) writing its state back to its own config file using a couple of measures:

1. Splits sentinel and server configs into a volatile and static (included) files
2. Writes a script that updates the running Sentinel with monitor configuration

In my testing, this results in idempotent configuration that should survive changes. It's not perfect -- it won't remove monitors that are no longer around, but it's a step in the right direction by not always rewriting sentinel's config file (or a sentinel-managed redis server config that is managed by sentinel).

This might be enough to fix #22?

[DavidWittman]

Thanks @joshbenner! These changes are excellent and have been sorely needed in this role for quite while. I'll do some testing around this PR and add it to the 2.0 milestone.

[tleguern]

Do you know when this PR can be fully tested and merged and is there something I can do to help you with it? This change is watched with interest at my job.

[Rylon]

@DavidWittman +1 for merging, we could really use it on my team. Any idea on the timeframe?