Exposing ability to mount servlets at sub-context-paths [IMMUTANT-253]

We no longer use RingServlet, but either a reified Servlet instance
wrapping a Ring handler or a proxied Servlet that sets the TCCL
appropriately before invoking the servlet's service method.

This is to support the mounting of Pedestal servlets comprised of
multiple "interceptors" rather than a single Ring handler.
Virgin pedestal-service app [IMMUTANT-253]
Mount the pedestal app in Immutant [IMMUTANT-253]

Currently, only mounting at the root context is supported.
Add an integ test for the pedestal app [IMMUTANT-253]