Add SG-specific cbgt cfg implementation

Sync Gateway is currently using cbgt's bucket-based config storage (cfgCB). There are possible optimizations based on the fact that Sync Gateway can be expected to already have a connection to the bucket, and a full DCP stream (caching stream) running.

A custom config manager would have the following enhancements:

1. Use Sync Gateway's existing gocb bucket connection. This avoids the authentication issues currently present for cfgCB (no x.509), but also simplifies initialization.
2. Check for the cfg key (or keys) when processing the caching feed, and use that to trigger config change notifications, instead of running a separate DCP stream for the vbucket. Also avoids cfgCB's dependency on cbdatasource.
3. Need to support Stop() on database.Close()