Investigate using firestore multi databases for data segregation

[hongalex]

With the introduction of Firestore multiple databases, we want to investigate the feasibility of introducing the capability of using multiple databases in Open Saves.

[hongalex]

Firestore multi-db requires creating named database instances. A new datastore client would be required to connect to each instance. Currently open saves only connects to a datastore via a single client. It would require some design changes to support connecting to individual databases. Such changes would include: finding a way to denote which database a Store / Record should live in, and a list of stores to make the connections.

In addition, there's a problem of scaling. Each datastore client instance created results in some level of network overhead, as each maintains it's own connection by default. This can be mitigated by sharing a connection pool across clients, but this can clash with the goals of true multi-tenancy.

Lastly, there's a question of how to isolate existing records into multiple databases. Approaching this directly would require some level of downtime to migrate records.