Consider supporting additional database backends for clustered operations

[ivan-tkatchev]

Of the currently supported storage backends, only FoundationDB is suitable for high-availability, clustered operations.

This is a strange choice as FoundationDB is a niche and poorly supported product with many bugs. For examples, see this (non-exhaustive) list of segault bugs: https://github.com/apple/foundationdb/issues?q=is%3Aissue%20state%3Aclosed%20%20segfault

Please consider supporting other clustered key-value stores for HA operation.

MongoDB is a good and stable choice. Etcd is also.

[mdecimus]

Stalwart can only support key value stores that offer transactions. There is a PR already to support etcd though, but MongoDB is not an option for Stalwart.