Fixes fd leak in leasedb (ticket #2015)

Brian correctly diagnozed this issue and suggested the fix in Weekly Dev
Chat of 2013-07-09. This patch has been manually tested by Zooko using his
fdleakfinder and a unit test has been added to ensure that database connections
are properly closed. Without this patch, the 1819-cloud-merge branch uses
more than 1020 fds and will start failing on operating systems with a low fds
limit.

The portions affecting leasedb.py were written by Zooko and Daira.