General

As part of the effort to future-proof Relay, this PR updates the
Healthcheck Actor to work with standard
Futures
instead of the former futures crate. It also moves away from
actix internally.

The bulk of the changes are to the healthcheck.rs file with minor
changes to other files to make it work with the reaming system. Some
code that is needed to interface with the current system can be removed
once the remaining system has been updated as well.

Design Choices

• Moving away from Actix lead us to introduce our own message handle
  loop in healthcheck.rs using tokio. Note that this does currently
  not allow for concurrent execution, which was deemed sufficient for
  the Healthcheck service for the moment.
• The Healthcheck service does not only need to handle the IsHealthy
  messages but also handle a Shutdown message. The special thing about
  that message is that in the current system the Controller can just
  send out Shutdown messages to all actors without needing to know
  anything about the internals of the actors. This can be achieved
  through a
  watch channel.
  The Controller has the sender and services (subscribiers) have
  clones of the receiver. There is a second message handle loop inside
  the Actors that receives the Shutdown message and forwards it to the
  primary message handle loop. This will be revised in a follow-up PR.
• For now, the actix SystemRegistry remains in place, however for that
  to work with the Tokio runtime a copy of the System needs to be
  available in each Tokio runtime thread. This is achieved by using the
  on_thread_start,
  and can be removed once actix is gone.
• To store the address to the Healthcheck service we currently use a
  lazy_static, a minimal viable Registry of sorts.

Future Steps

• With the switch from Futures 0.1 to Futures 0.3 on the horizon, it
  might be nice to refactor the system by changing: futures -->
  futures01 futures03 --> futures
• The current design of the Healthcheck service only allows for
  limited parallelization. As such, it might be interesting to use
  channels internally to protect the internal resource and allow for
  greater parallelization.
• A good choice for the next upgrade is the Upstream actor. This will
  require us to circle back and remove the hardcoded bool in the
  struct Message<T>.

This is a resubmit of #1349