Collections: Get Collection ID command

Collections design requires we add a bucket level getCid command.

The input to the command is a string which describes the 'path' to the collection using the same format as the N1QL collections path spec.

E.g. "scope.collection"

The command also supports _default scope by omission

• ".collection" is the collection from the default scope.
• "." is the default collection in the default scope and returns 0 (or unknown if default dropped).

The command will use the last manifest receievd from ns_server to perform the collection-ID lookup.

Failures:

• Collection unknown
• No manifest, ns_server has yet to give the bucket a collection config
• EINVAL for other errors, e.g. path is ""