Track high-connection-count users

[adehnert]

We should have a cronjob that runs every few minutes(?) and dumps of list of all users with more than 50 connections(?) to a file on each sql.mit.edu machine. This would help us know which users to investigate for compromises and/or talk to about their connection count being too high, and would supplement and make more useful our nagios user connections alert and forthcoming munin monitoring of it (#346).