index: cleanup internal variables

index: cleanup internal variables

Move away from hard-to-read alllowercase naming and favor
snake_case or separated-by-dashes.

We'll keep `--indexlevel' as-is for now, since it's been around
for several releases; but we'll support `--index-level' in the
CLI and update our documentation in a few months.

We'll also clarify that publicInbox.indexMaxSize is only
intended for -index, and not -watch or -mda.