Support JSON column containing measurements and counts

https://github.com/azavea/osmesa/issues/140 simplifies the underlying database schema so that individual measurements and counts are stored in a JSON column rather than as individual columns. osmesa-stat-server needs updates to read from this schema.

Review the linked osmesa issue #140 first to understand the revised database schema, then inspect osmesa-stat-server to find where measurements and counts are read. Done means the server reads individual measurements and counts from the JSON column instead of the former individual columns.

Written by the indexing model from the issue text.