Issue: SPR-15190

Issue: SPR-15190

This commit removes the use of SocketUtils#findAvailableTcpPort in
favor of letting servers pick a dynamic port by specifying port 0.

This should make integration tests more stable because the port is
chosen at the place where it needs to be used. It gives servers a
chance to try to open a socket on some port and start using the socket
if successful.

This commit introduces support for running multiple HttpHandler's under
distinct context paths which effectively allows running multiple
applications on the same server. ContextPathIntegrationTests contains
an example of two applications with different context paths.

In order to support this the HttpHandler adapters for all supported
runtimes now have a common base class HttpHandlerAdapterSupport
which has two constructor choices -- one with a single HttpHandler and
another with a Map<String, HttpHandler>.

Note that in addition to the contextPath under which an HttpHandler is
configured there may also be a "native" contextPath under which the
native runtime adapter is configured (e.g. Servlet containers). In such
cases the contextPath is a combination of the native contextPath and
the contextPath assigned to the HttpHandler. See for example
HttpHandlerAdapterSupportTests.

Issue: SPR-14726