[5.4] Add support for PhpRedis

[tillkruss]

I messed up the branch and didn't know how to recover. This is the original PR: #14850

Followup from laravel/docs#2500 to add PhpRedis support to Laravel, because it's significatly faster than Predis.

This is a proof of concept. Feedback/suggestions are greatly appreciated.

• Illuminate\Redis\Database is now an abstract class.
• PredisDatabase and PhpRedisDatabase were introduced, extending Illuminate\Redis\Database
• Laravel uses Illuminate\Contracts\Redis\Database instead of Illuminate\Redis\Database in most places.
• Illuminate/Cache/RedisStore and Illuminate/Cache/Repository now check key existence using null or false.
• Laravel will use Predis as default unless the config value database.redis.client is set to phpredis.
• Predis connection parameters are mapped to the slightly different Redis and RedisCluster parameters.
• I've pointed the core alias for redis to the abstract Illuminate\Redis\Database class. I'm not sure if that's correct, or if we need a Redis Database Manager or similar.

To test/use this:

• Rename (or uncomment) Redis facade in config/app.php.
• Add 'client' => 'phpredis', to redis array in config/database.php.

[GrahamCampbell]

I've set the target of this PR to 5.4 due to this breaking change.

[taylorotwell]

Is it possible for us to get integration tests on this?

[taylorotwell]

We'll also need some docs on the master branch of the docs. Thanks!

[tillkruss]

Sure thing. I'll work on that this weekend. I'm slammed with work at the moment, I'll do this soon.

[philbates35]

This looks like a typo, should be $this->subscribe(...)?

[GrahamCampbell]

Please send a PR. :)

[tillkruss]

Done.

[dzgrief]

Now I am using laravel 5.3, is this feature can support 5.3 ?

[tillkruss]

It's coming in 5.4.

For 5.3 you need to use this package https://github.com/tillkruss/laravel-phpredis

[halaei]

@tillkruss Can you please tell why you made this change:

Illuminate/Cache/RedisStore and Illuminate/Cache/Repository now check key existence using null or false.

I think it should be reverted.