Switch the configuration plumbing for pools and sources to be pull-based
rather than push based.
In other words, rather than defining them in the `init` event,
you now need to supply them to the new `get_egress_pool` and
`get_egress_source` events.
Data is cached by default for 1 minute. This allows for new sources
to come into being on-demand, and for data to age out and change
over time, without requiring that the server be restarted.
This commit updates the reference section, but there is some content
in the user guide that refers to the old style of configuration that
will need to be updated.
refs: https://github.com/KumoCorp/kumomta/issues/13
Wez Furlong
Mike Hillyer