Allow multiple BaseCouch instances to share a bucket (AKA "shared-nothing")

[snej]

For scalability, we need to support any number of BaseCouch processes sharing the same Couchbase bucket.

Here is a design wiki for this

Currently this almost works; the exception is the _changes feed. It may need to wait for changes to the database, and the mechanism it uses for this is based on in-process Go notifications. So a _changes feed request being served by one process will wait forever without noticing a database change made by another process.

To fix this we need to hook into the Couchbase Server 'TAP' feed, or else some new mechanism TBD.

[snej]

The changes on the 'scalable' branch implemented this. It's now been merged into master.
There may still be race conditions to discover and fix.