Redis pubsub support

[yii-bot]

This issue has originally been reported by @nineinchnick at https://github.com/yiisoft/yii2/issues/1932. Moved here by @cebe.

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When (P)SUBSCRIBE is called in Redis a client is not supposed to execute any more commands except for (P)(UN)SUBSCRIBE. He then should listen for incoming messages.

This can be achieved by making parseResponse() public. Also, either it has to return null on timeout instead of an exception or throw a different exception so it can be caught. Or at least one with a different code.

I require this in my queue implementation that I'm working on for yiisoft/yii2#492.