Nhse d34 nhskv.i33 token

[martinsumner]

Add support for a token manager and token session process to act as a requestor for tokens, and warp riak_client calls within a session.

This is then used to be provide a stronger conditional PUT API to allow for check-and-set operations that will behave reliably when the cluster is healthy, as well as in simple failure scenarios and operational changes. The CAS will fall-back to eventual consistency in extreme failure scenarios.

[martinsumner]

Looking at the results of a full performance test, the overhead of performing basic_consensus for a proportion of PUTs is not large