Redis refactor

[mbland]

This moves all the former lib/redis-client.js code to the new lib/redis module. Several auxiliary and utility classes have been split out into separate modules. In particular:

• The Keys module centralizes the creation of Redis keys, which helps document how we're using them.
• The PromiseHelper module eliminates a lot of boilerplate wrapper code for callback(err, data)-style Redis API functions.
• The AutocompleteIndexer and TargetIndexer interfaces now use the standard addLink/shouldReindexLink/removeLink, which enables the RedisClient implementation to treat them more uniformly and eliminate duplicate code.

[coveralls]

Coverage increased (+0.04%) to 98.194% when pulling cadf31b6af63084d202e8871a6358801eb03c4c1 on redis-refactor into c5a1572072a13f4270fea22714eefdfe607b94c3 on master.

[mbland]

I had to mark some duplicate code warnings as Invalid, since the methods in questions were thin wrappers around helper functions. Merging.