Implementation of Nette/Caching/BulkReader

[martinsifra]

At this moment, there is an implementation of Redis mget (get multiple keys via one command) with interface IMultiReadStorage. Latest version (2.5) of Nette\Caching has similar posibility in https://github.com/nette/caching/blob/master/src/Caching/BulkReader.php. The possibility has been there since v2.2.

This could improve reading from Redis and decrease an amount of get calls.

Some additional information why to use mget:

• benchmark (https://medium.com/@jychen7/redis-get-pipeline-vs-mget-6e41aeaecef)