Support upserts and deletes on metadata only

[lewismc]

After #270 we can consider a use case which demands we accommodate arbitrary upserts and deletes to metadata.

• same positional (track) data, same measurement data, new metadata
• same positional (track) data, same measurement data, update existing metadata
• same positional (track) data, same measurement data, delete existing metadata

[lewismc]

See versioning scenarios for details on the ingestion scenarios. I will create a follow-on ticket for documenting this more thoroughly on the wiki.