Fixes #109

The web module doesn't really need to depend on tomcat and all of
the Spring Boot web stack, but users need a way to grab that stuff
quickly if they want it (hence the new starter).

Also removed all spring-boot-starter dependencies from core and
context modules.

Functions with Flux and Message (as well as POJOs and Flux of POJO
which were already supported) should now work if they are created in
an isolated class loader. Preconditions:

* The class loaders must have the reactor-core (and reactive-streams)
shared between the app and the function. Practically speaking this means
there has to be a parent class loader with just reactive types, and
sibling children for the app and the function. This is not a new
requirement (it was needed for Flux of POJO anyway).

* Message types are handled reflectively, so they don't have to be in a
shared class loader. But they do have to be  on the class path on
both sides (obviously).

Otherwise you get the wrong result when you go back and ask later
(but only if the bean is a singleton with no bean definition).

If FooHandler extends AzureSpringBootRequestHandler apparently
Azure cannot extract the generic types Foo and Bar.

User can extend SpringBootApiGatewayRequestHandler instead of the
generic SpringBootRequestHandler. It ties the code to AWS and the
API Gateway, but at least it supports the incoming data fully.

Fixes gh-111, closes gh-136

Use of BeanProcessor to catch a bean before it is used is a bit
agricultural these days. SmartInitializingSingleton is better and
frees application logs from one more annoying INFO log on an
early instantiation

2018-01-04 14:17:05.930  INFO 23472 --- [       Thread-0] trationDelegate$BeanPostProcessorChecker :
  Bean 'org.springframework.cloud.function.web.flux.ReactorAutoConfiguration' of type
  [org.springframework.cloud.function.web.flux.ReactorAutoConfiguration$$EnhancerBySpringCGLIB$$8d4844e]
  is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)

With this change there are now 2 choices for web endpoints. The
stream servlet binder is useful for multi-binder use cases
(e.g. HTTP -> message broker).

Fixes gh-128