Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

This commit refactors the `ClientHttpRequestFactory` into an
`ClientHttpConnector` abstraction, in order to reflect that
`ClientHttpRequest`s only "exist" once the client is connected
to the origin server.

This is why the HTTP client is now callback-based, containing all
interactions with the request within a
`Function<ClientHttpRequest,Mono<Void>>` that signals when it's done
writing to the request.

The `ClientHttpRequest` contract also adopts `setComplete()`
and promotes that method to the `ReactiveHttpOutputMessage` contract.

This commit also adapts all other APIs to that change and fixes a few
issues, including:

* use `HttpMessageConverter`s instead of `Encoders`/`Decoders`
* better handle type information about request content publishers
* support client cookies in HTTP requests
* temporarily remove the RxNetty client support