Add "upsert" option for keys about hecate HOT 2 CLOSED

Unfortunately a pure UPSERT operation isn't possible due to the transactional, non-locking nature of deltas.

A conflicting key can be quickly found and an error thrown but an upsert operation without the client knowing the version of the given feature on the server would result in the ability for multiple clients to push conflicting changes simultaneously.

That said, a two pronged approach can simplify the UPSERT style operation. The first is a new search by key query endpoint, which will return the feature for a given key query. Ref: #78

The second is to have a more standard JSON response for upload errors that where possible returns the feature that caused the error. This change is pending.

from hecate.

Error by id has landed here: #80

from hecate.