Review use of `/tmp` and public secondary databases across invocations

[refset]

I just added ATTACH DATABASE support such that Play can now read from any public S3 bucket. Behind the scenes there's a shared object store cache which may 'leak' previously attached databases from earlier invocations. XT doesn't currently offer an SQL APIs that could interrogate this, but it's something to mindful of.

Similarly, users are able to specify !Local directories in their ATTACH config, although shouldn't need to (since in-memory works fine), and nothing should get written (compaction is disabled).

Further, ATTACH DATABASE is effectively read only right now, but if BEGIN ever supports a DB param or USE DATABASE gets implemented (etc.) then we may want to lock things down further.

[refset]

Have just added a hook to clear the /tmp/disk-cache dir between invocations