Provide mechanisms for cleaner lumbermill shutdown.

- Allow draining and completely of in-flight requests
- Refuse requests after shutdown has been requested.
forgot to update the tests
Move connection recycle startup into server.Run()
Add signaler interface to avoid state leak of shutdown chan
WIP, but a bit confused about what's going on...
Cleaner shutdown of lumbermill with end to end tests

 - Test sets up a mock influxdb which waits for 2 seconds
   before responding
 - Test fires 100 batches to /drain
 - Test asserts that all batches were posted
 - Test attempts to assert that all 1000 points were delivered
Remove Signaler and use io.Closer instead
Ensure Connection: close sent when shutting down.
Merge pull request #10 from heroku/cleaner-shutdown

Cleaner shutdown