(TK-19) Add `service-symbol` and `get-services`

This commit does the following:

* Adds a function `get-services` to the `Service` protocol.
  This allows services to get a sequence of all of the
  other services in their app, which will allow for more
  sophisticated behaviors / interactions between services.

* Adds a function `service-symbol` to the `Service` protocol.
  This function returns the clojure symbol for the service
  definition (assuming that the service was defined using
  `defservice`).  This will allow us to, e.g., use service
  namespaces/names in config files, and, when combined with the
  `get-services` function above, to write services that are
  capable of looking up other services based on their
  symbol.
Perhaps this could be expressed more cleanly using a schema instead of a post-condition.