Document performance considerations

[lesv]

Proposal

Emblem's decision record for session state should mention the latency characteristics of the various options as well as cost.

Problems this will solve

I'm not convinced we should be pointing customers at GCS for session store. #258 Firestore, which is built on top of Spanner is a lot faster than Datastore and makes some sense in this environment. Choosing Firestore would have required enabling the API, but would otherwise be of minimal impact on the user.

Alternatives

Small users should be considering Firestore. Larger users may be interested in Redis / Memcache or Postgres / AlloyDB / Spanner or Bigtable.

Additional context