TCG-2460 - Automatically refresh frequently used keys before they expire

[guywithnose]

What does this PR do?

TCG-2460

Checklist

• [ ] Pull request contains a clear definition of changes
• [ ] Tests (either unit, integration, or acceptance) written and passing
• [ ] Relevant documentation produced and/or updated

[chadicus]

In my opinion the functionality you are adding should not be in the Predis class, but instead be part of a custom ClientInterface instance.

[coveralls]

Coverage remained the same at 100.0% when pulling f7a7a21 on guywithnose:master into 0e8a29e on traderinteractive:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 3114e9b on guywithnose:master into 0e8a29e on traderinteractive:master.

[guywithnose]

I pulled out the locking part, because as @chadicus pointed out that may work better at a different abstraction level. The part that measures runtime still needs to be here, so I left that.

[guywithnose]

@chadicus Here is where I moved the locking part: https://github.com/traderinteractive/lib-predis-anti-stampede-php/pull/1

[chrisryan]

This looks good to me. I have two thoughts/considerations depending on how we want to approach backward compatibility.

1. Should the default refreshPercent be 0 so the behavior does not change unexpectedly in cases where the refresh is not desirable for some reason?
2. There could be an argument that these changes should be in a wrapper or extended class which would avoid the first issue.

I think the first issue would be easy enough but understand it would require us to do more work than just upgrade the library. I am open to other thoughts/opinions.

[guywithnose]

I set refreshPercent to default to 0. That should make it fully BC