Modify cache redis backend

[pantaovay]

Changes

1. Modify cache redis backend, delete _PHCR
2. Cache redis backend supports "redis" client option

Why

1. The redis cache backend use a special key _PHCR to store all keys used by cache. There is a issue: why the backend cache redis add the special key(statsKey) _PHCR #10905 .According to @Green-Cat ,it is disabled in 2.0.x branch which is not true. If use _PHCR, every time we set a cache, we have to run sAdd and set commands. Store all keys in _PHCR will cause it expands quickly and this will influence performance.
2. The cache backend doesn't support redis option which means that we can not pass a redis connection. So if we use modelsCache and modelsMetadata, there will be two redis connections. So I modify this part and let redis option be supported, and through the lifetime of a request we can use one redis connection.

[pantaovay]

@ReenExe @sergeyklay

[sergeyklay]

Looks good to me

@andresgutierrez

[tylercb]

+1

[Green-Cat]

👍

[sergeyklay]

@pantaovay 2.1.x is best place for this. Also could you please update CHANGELOG.md?

[pantaovay]

@sergeyklay I am working on it

[sergeyklay]

@pantaovay

[sergeyklay]

Close in favor of #11608