This can be reverted in 1.1. The compiler relies on being able to
bind itself to @ConfigurationProperties and the APIs changed for that
in Spring Boot 2.0.
Fixes gh-177
If an isolated function doesn't have Message in its classpath, we
will never actually need to instantiate that class. This change
makes sure we check first.
This results in a better experience for users because the consumer
that they write is only applied to a Flux that is subscribed to
by the framework once. It gives better control over the flow of
foos, e.g. if some component wants to subscribe on a thread.
When a Supplier<Flux<Foo>> is composed with a Function<Foo,Bar>
the resulting handler (supplier) should have Flux as its output
wrapper still (the most general output wrapper type in the chain).
A Spring Boot 2.0 app should behave the same as a Spering Boot 1.5
app with this change. The key thing was to change the return type
of the FunctionController and move the computation of single
valuedness for the output there (instead of trying to do it in the
return value handler, which isn't used in Spring 5).
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.
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
