[ENG-2104] oai-pmh using FormattedMetadataRecord

[aaxelb]

• add a parallel path for OAI-PMH
  • add oai_dc metadata formatter
  • alternate implementation of /oai-pmh/ endpoint
  • currently unused by default -- enable for a given request with ?pls_trove query param, or for every request with environment variable SHARE_LEGACY_PIPELINE=0 (will also disable other ShareObject-based things)
  • when ShareObjects die, this will become the default/only path
• refactor metadata formatter tests
  • set of test cases that should easy to share/extend for all metadata formatters into the future
  • for future formatters, extend tests.share.metadata_formats.base.BaseMetadataFormatterTest with expected outputs and success criterion
• fix a bug in share.util.graph.MutableNode -- allow traversing reflexive many-to-many relations

jira: ENG-2104

[coveralls]

Coverage increased (+0.4%) to 80.045% when pulling 71e4797ba66764205ccb0cf7281362f37376a658 on aaxelb:eng-2104--oai_dc-formatter into 6c356c31bb1d0b521226334f6030762de2a97ec3 on CenterForOpenScience:develop.