OTP 24 Support

[martinsumner]

Intention is to start work on OTP 24 support in January.

Couple of things which need to be considered:

• Should lager be replaced with logger?
• Should webmachine be replaced with cowmachine? Is there a real driver for this?
• Naming. develop-3.1/3.1.n conflicts with internal naming conventions at a key riak user. develop-3.2/3.2.n or develop-4.0/4.0.n?
• Should leveldb be removed from role in riak_core (cluster metadata AAE), potentially enabling leveldb to be deprecated/removed. Lack of confidence wrt changing leveldb going forward. Issues with Graviton.
• Should this be an opportunity to deprecate other features, and slim down dependencies and reduce feature conflicts?