Sentinel persistent

[Xon]

On-going usability improvements to sentinels and slaves.

• Allows various options (ie persistence) to be configured independently of what is used for the sentinel/master/slave
• Pass the connected master redis instance to $slaveSelect callback (backwards compatible with previous signature)
• Prevent an Error throwable when null is passed to _applyClientOptions (php +7.0), as this is not caught by the Exception catch filter.
• Update _applyClientOptions to respect the persistent setting it has vs what client options is requesting.

[Xon]

@colinmollenhour I'm still fiddling with how to present extra sentinel/slave options in a way which isn't confusing but allows the code to be as consistent as possible with how options for sentinel/slave/master are presented.

[Xon]

@colinmollenhour do you have any feedback on this change?

I haven't had any issues so far with it.

[colinmollenhour]

I have tested it briefly and it worked fine. I think this is about ready to merge into master. Any last requests? :) Thanks for the help, btw!

[Xon]

Nope, nothing related to this PR.