Add support for setMaxConnectionAge in db connection
Create new conn-lifetime setting

This restores the original behavior for conn-max-age (for
backwards-compatibility reasons), and creates a new option called
`conn-lifetime` that governs how long idle/active connections stick
around.
Document db pool settings
Change default db conn keepalive to 45m

This better matches up with the standard firewall or load balancer
idle connection timeouts in the wild.
Merge pull request #587 from grimradical/pullreq_586

Add conn-lifetime setting to DB pool