This commit
* adds new request and response data types for schema gRPC calls
* adds fmt.Stringer implementation to cursors.FieldType
* adds APIs to sort a slice of MeasurementField values,
* upgrades the gogo protobuf package to v1.3.1, which
includes improvements to serialization.
These APIs require a measurement, permitting an additional optimization
to reduce the search space against the TSM index. Specifically, the
search key prefix is extended from `org+bucket` to
`org+bucket,\x00=<measurement>`
* MeasurementNames
* MeasurementTagKeys
* MeasurementTagValues
* Adds an api to the models package for efficiently parsing the
measurement tag (\x00) from a normalized series key
Stuart Carnie