db_poller isn't getting adjusted

At the time we call def reconfigService(self, new_config):
self.config.db['db_poll_interval']  has the new value already assigned,
so this comparison is never true
https://github.com/buildbot/buildbot/blob/master/master/buildbot/master.py#L303
if (self.config.db['db_poll_interval']   !=
new_config.db['db_poll_interval'])

and the db_poller isn't getting adjusted, changed the reconfigService
implementations to store initial configuration in object attributes, and
compare those attributes to new_config.
code review fixes

- change configured_url -> configured_db_url
- put all of the config-handling code in reconfigService
- add release notes
change tests to correspond
Merge branch 'pull639'