use CockroachDB enums in more places in database schema

There are a few places in the schema where we'd like to use CockroachDB enums, but we were prevented by cockroachdb/cockroach#57411 / cockroachdb/cockroach#58084. This was supposed to be resolved in v21.1. So with #195 in, we should be able to switch to the enums. (This is worth confirming before doing too much work here.)

Examples where we want to use this:

• "Instance" table, "state" field.
• "Disk" table, "disk_state" field.
• "Saga" table, "saga_state" field.
• "SagaNodeEvent" table, "event_type" field.

I skimmed the schema to look for other candidates and I think that about covers it.

I expect this change will involve, for each of the above cases:

• update the schema SQL to create the enum (many of these have a commented-out version already -- that needs to be compared to the actual Rust enum to make sure these are in sync)
• presumably there's some change we need to make to the Diesel table! macro invocations, but I'm not sure what that looks like yet

I think that's it, but I'm not sure if there's something else we need to do on the Diesel side.

First confirm that the CockroachDB v21.1 limitations in cockroachdb/cockroach#57411 and #58084 are resolved, then inspect the schema SQL, the commented enum definitions, the corresponding Rust enums, and the Diesel table! macro invocations. Done means the Instance.state, Disk.disk_state, Saga.saga_state, and SagaNodeEvent.event_type fields use synchronized enums throughout.