Enhance json mode statistics to better support per-dhandle statistics

Currently when enabling JSON output mode for statistics, and dumping stats per collection, the name of the table isn't included in per-table output, so it's not possible to map the set of stats back to a particular table.

An example wiredtiger_config option string that generates the unexpected output is:

statistics=(cache_walk),statistics_log=(wait=60,sources=[file:],json=true)