Change name of MetadataService to DatasetService

[david4096]

The usage of the word "metadata" in the API is a loaded one, and perhaps we can improve the understandability of the protocol by renaming the Metadata Service to the Dataset Service, that presents methods that organize genomic data into datasets.

This change in file names would present no change at the API level, servers still present the same HTTP endpoints. This change would minorly alter documentation and changes the namespace used by modules that depend on the schemas.

[mbaudis]

+1

[kozbo]

+1 I find the current naming confusing.