(TK-3) Add support for multiple servers

Add support for multiple servers to the jetty9 webservice. Previously, a user would define the webserver key in the configuration file as a map of configuration data, which would be used to configure a single Jetty9 server. This syntax is still valid. However, now the user can also define the webserver key as a nested map, where each key is the id of a server and each value is a map of configuration data (of the same form as before). With this information, the Jetty9 service will now initialize, start-up, and shutdown multiple servers if so specified by the user.
So I think we currently don't have *any* test coverage of the other `-to` functions besides `add-ring-handler-to`?

I don't think that we necessarily need to test both flavors super-duper exhaustively; it's fine to pick one or the other for us to have the level of test coverage that we currently have.  But for the other flavor, we should at least have some super minimal basic tests for each function.  So I think we could get there by adding one test each, the most basic possible, for `add-servlet-handler-to`, `add-war-handler-to`, etc.