The controller's client is instantiated in the
`control::destination::background` module and is tightly coupled to its
use for address resolution.
In order to share this client across different modules---and to bring it
into line with the rest of the proxy's modular layout---the controller
client is now configured and instantiated in `app::main`. The
`app::control` module includes additional stack modules needed to
configure this client.
Our dependency on tower-buffer has been updated so that buffered
services may be cloned.
The `proxy::reconnect` module has been extended to support a
configurable fixed reconnect backoff; and this backoff delay has been
made configurable via the environment.