Allow retrying more than GET

[hexylena]

extracted retry logic from _get into a retry method.

• untested (even locally) just want to discuss implementation
• should retry be _retry?
• renames existing methods. Should the old names be retained as 'legacy' API? Should they include a deprecation notice somehow?

xref https://github.com/galaxyproject/ephemeris/issues/98#issuecomment-410627238