Some index statistics related improvements

* Increase the number of histograms in the default config, but don't change it for tests so that they continue to run.
* Add a config option to control how often scanning commits and pauses. By default, this continues to do the whole scan in a single transaction.
* Move warning about missing / out-of-date statistics into the service itself, where it can avoid duplicating the same warning.
* Add another config option in the same format to enable background scans in these warning situations. By default, this one has a 10% duty cycle: 1 sec on, 9 secs off. I don't have any real basis for those numbers.