Flexible connections for sentinel support

[benbuckman]

This supports redis-sentinel-client. The changes are mostly cherry-picked from tanguylebarzic's fork and our/DocuSignDev's fork.

There is a discussion about how to support sentinels here: #302
The idea of supporting pluggable connection managers is mentioned there, and may be more robust. I'm submitting this PR so the changes needed for my sentinel implementation are documented.

Thank you.

[brycebaril]

Working on digesting this. One thing I'm hoping to do with pluggable connection managers is deal with the RedisClient options bloat that has been happening. We keep adding new options, which is a bit of a maintenance and documentation nightmare.

This patch adds two (disable_flush and allowNoSocket right? -- also, notice the casing inconsistency) new options that aren't documented, and may not provide useful behavior when the 3rd-party client that uses them isn't in use.

[philjackson]

Has there been any movement on this? Would be really good to get something into NPM that works.

[justinfreitag-zz]

bump

[analytically]

👍