event enums: Rename to conform with oasis-core

[mitjat]

This came up in https://github.com/oasisprotocol/oasis-indexer/pull/246/files#r1048033749

Unlike the indexer, oasis-core does not have an enum of all event types. Instead, events are represented in a nested struct that is used as a union type. So to represent a Transfer event, for example, oasis-core use an object like Event { Staking: { Transfer: { ... } } }. There's no explicit enumeration/names for event types. The indexer, OTOH, needs an enum so that clients can somehow say "give me only events of type X".

Enums were named somewhat arbitrarily. They were named after the last element of the "path" in the above composite object. In addition, the first element of the path (in practice, the originating tendermint app) was saved as a separate backend column in the DB, but it only ever makes sense to expose both backend and type at the same time, or neither.

This PR changes the enum value scheme as follows:

• backend and type are merged into a single field, simplifying handling and understanding
• JSON/CBOR names of fields are used, rather than Go-internal struct field names
• When the "path" from the top-level Event to the leaf event struct has more than two elements, all elements are included in the enum name (e.g. staking.escrow.take)

[Andrew7234]

Thanks!

[pro-wh]

k

[aefhm]

(non-blocking) We don't foresee searching by backend?

[mitjat]

(non-blocking) We don't foresee searching by backend?

I can't think of a use case, no. And if we need to for some niche reason, we can still do prefix matching on type.