Connecting with the rediss: protocol won't work if TLS isn't enabled,
and the error message is a generic "connection timed out".

… is not available

Rather than non-durably in process memory.  This solves two issues:

 1. It allows for production use without Redis as long as certain other
    requirements are met.  I'll be documenting these and relaxing the
    Redis requirement in a subsequent commit.  This is motivated by
    CDC's deployment.¹

 2. It makes development more like production, which is important as the
    difference wasn't obvious and led to strange behaviour, e.g. across
    server restarts.²

I knew the transient in-memory store was not ideal when I wrote it, but
it was very expedient! and I didn't want to address it further at the
time.  This now rectifies that.

The configuration of the keyv library with a key namespace ensures that
no key name migration is needed with this change.

¹ <nextstrain/private#94>
² <#581 (comment)>

This is made possible by the previous commit which switched the
non-Redis fallback from an in-memory store to an on-disk SQLite store.

As a nice side-effect, we can now set REDIS_REQUIRED=true in development
mode (i.e. not the default) to more easily test the production mode code
path.

Related-to: <nextstrain/private#94>