Issue: SPR-14940

Issue: SPR-14994

Issue:  SPR-14986

Issue: SPR-14972

Issue: SPR-14940

Due to a few changes in `WebSocketServerFactory` and `Session` API, our
`JettyRequestUpgradeStrategy` and `JettyWebSocketSession` needed to
adapt. As of 9.3.15+ and 9.4.0+, some reflection is required to support
previous versions.

Spring 5.0 supports Jetty 9.3 and 9.4.

Issue: SPR-14940

Issue: SPR-14875

Issue: SPR-14990

Issue: SPR-14985

Issue: SPR-14976

Issue: SPR-14957

Issue: SPR-14361
Issue: SPR-14987

Issue: SPR-14969

Issue: SPR-14901

Fixes SPR-14901

Issue: SPR-14899

Issue: SPR-14988

Issue: SPR-15527

Issue: SPR-14991

This reverts commit 472126c0.

Issue: SPR-14940

This commit ensures that POST/PUT requests sent by the Netty client have
a Content-Length header set.

Integration tests have been refactored to use mockwebserver instead of
Jetty and have been parameterized to run on all available supported
clients.

Issue: SPR-14860

Issue: SPR-14527

Issue: SPR-14527

Issue: SPR-14527

This commit changes RouterFunctions.toHttpHandler to return
HttpWebHandlerAdapter (instead of HttpHandler), so that WebFilters and
WebExceptionHandler can be applied to it.

Issue: SPR-14979

This commit introduces a new method on WebClient: filter, which takes an
filter function, and returns a (filtered) WebClient.

Issue: SPR-14961

- Add 4xx/5xx status code check to ClientResponse.bodyToMono and bodyToFlux.
- Removed WebClient.retrieveMono and retrieveFlux.

Issue: SPR-14977

Issue: SPR-14975

ChannelFutureMono renamed to FutureMono

In the latest reactor-netty there is renaming
of ChannelFutureMono class to FutureMono. Thus
build fails on the latest master.

Issue: SPR-14533

This commit adds the ability to serve Resources (static files) through a
RouterFunction. Two methods have been added to RouterFunctions: one that
exposes a given directory given a path pattern, and a generic method
that requires a lookup function.

Issue: SPR-14913

Issue: SPR-14467