Improve async support in core module

[Sven Döring]

Thanks to @Lukáš Křečan the springmvc module gained support for async requests. Even the CachingServletInputStream has been refactored for better async support.

The core module, however, does not validate in an async manner. I don’t think this is totally necessary but at least the request body validation shall support reading the bodies asynchronously.

The Jackson ObjectMapper supports async requests since v2.9.
The InputStreamBody might use the feeder in case of async requests.

See: https://stackoverflow.com/questions/10862088/reading-json-stream-without-blocking