Fix stats bug
[server] Show intervals starting with the most recent in metricz
A couple of forgotten file closes
Store dimension tables outside of the metadata JSON

I'm storing the string slices in gzipped gob files; one per string
dimension. This avoids having a massive and unwieldy db.json.
[gumtool] Teach clean about dimension table files
[gumtool] A one-off utility for migrating to new dimension table format
A configuration option to raise the open files limit

This is necessary because gumshoedb opens a lot of files. There can be
thousands of segments, and each one is two file descriptors (one a
regular open file, one for the mmap).
Revert "[gumtool] A one-off utility for migrating to new dimension table format"

This reverts commit 12119ba2b4a6437929a9acd7390d93049c918aac.

I ran the migration on our prod DB and this is no longer needed.