Move database to temporary storage

[pinkwah]

We are running the ert storage database from NFS, which is slow.

Instead, at startup, copy the database to /tmp. Split up the Db fastapi depend into ReadDb and WriteDb. ReadDb simply reads from the /tmp database. WriteDb writes to the real database, then copies the db to /tmp when it's done. We might need to recreate the SqlAlchemy engine between runs, which is a bummer.

[oysteoh]

@ManInFez had this comment while discussing

I think having a snapshot created in temp on startup of ertviz, and then, on demand from the user, “refresh” the snapshot

so - it is like what you have stated here almost, and i think it sounds like a good approach. Let's deal with the read bottleneck first ( I argued yesterday about having the writes also to a tmp-db, but let's face one thing at the time... )