Use a faster query to retrieve report_timestamp

This was performing a correlated subquery with an ORDER BY clause on
reports in order to retrieve the latest report for each node, which is
slow and runs many times. Now we take advantage of the latest_reports
table to make fetching the latest report inexpensive. In testing, the
round trip time for /v2/nodes with 179 nodes went from ~700ms to ~33ms.
Merge pull request #633 from nicklewis/faster-report-timestamps

Use a faster query to retrieve report_timestamp