Better error handling for redis errors after connection.

This prevents the startup callback from being called twice when an error
occurs after startup.
Optional rollbar setup for logging errors.

Also improve startup process so the timeout callbacks don't get called
at all if the task we're timing succeeds.
Fix some odd Varnish integration errors related to chunked handling.

Basically, the chunked handling has some oddities, that I thought we
were already testing around, but it turns out some of our existing tests
started to behave differently when the testing environment was low on
memory. We now have some separate tests to more broadly test the
response chunk behavior.