Implement Support for DCP

Implement support for 3.0+ DCP as a replacement for the TAP protocol and TAP Client.

To be a suitable replacement, this has a few additional requirements to meet current TAP features:

• It must be able to use buckets with authentication/passwords in addition to the default bucket
• It must be able to target active data only, rather than active and replica data
• We must be able to split/filter to individual nodes from the client interface. It may also be useful for this to be at the vbucket level.

The use case for the latter is that in some cases, such a job may be split across many client nodes reading data from a cluster with many nodes. By allowing the application code using this client library for DCP to split along these lines, it can manage how the job is run for maximum parallelism.