Make CBS connection pool size configurable; default to sensible values

* Updated to latest go-couchbase with Dustin's patch to make the size of
  the socket pools configurable.
* Initialize these sizes to reasonable values (16 sockets, 0 overflow).
* Expose these as top-level config properties "maxConnections" and
  "maxOverflowConnections" so people can override them if they want.
Fixes #195