Adds some extra debugging information to make it clear that you are
running `significant_text`. Also adds some using timing information
around the `_source` fetch and the `terms` accumulation. This lets you
calculate a third useful timing number: the analysis time. It is
`collect_ns - fetch_ns - accumulation_ns`.

This also adds a half dozen extra REST tests to get a *fairly*
comprehensive set of the operations this supports. It doesn't cover all
of the significance heuristic parsing, but its certainly much better
than what we had.