Implements #945: monitor Redis key lengths

Using the 'keys' parameter with a list of key names (e.g. 'testkey')
in one Redis instance adds a metric 'redis.key.length' where list
lengths are reported using the LLEN command and they are tagged with
their names (e.g. tag 'key:testkey').
Checks that keys are entered as a list in config
Merge pull request #962 from DataDog/leo/rediskeyslength

Implements #945: monitor Redis key lengths