(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.
So... I started to look into this, and there are soooo many functions in this namespace that are already using pre/post instead of schema... I think it would be really weird to just port this one single function over to schema, and I don't think there's enough value to warrant the amount of effort to refactor the whole namespace right now... so I'd prefer not to do this.  I can create a ticket to reflect this work if you guys think it's worthwhile?