Change list reference bases from GET to POST

[david4096]

This PR changes the functionality of the list reference bases endpoint to a style more consistent with the remainder of the API. https://github.com/ga4gh/schemas/pull/647 https://github.com/ga4gh/server/pull/1309

• Reference ID's are now included directly in the request.
• The path for listing reference bases is simpler.

If the schemas are accepted the POM's need to be changed before this PR is merged.

[bwalsh]

+1

[kozbo]

@david4096 if you re-sync this I will merge :-) +1