Added parameter checks to NetClientImpl, CaOptionsImpl, FileSystemImpl, HttpClientImpl, HttpClientRequestImpl, SocketAddressImpl, WebSocketConnectOptionsImpl, added helper class Arguments for argument validation analogous to Objects.requireNonNull(), refactored parameter checks to use consistently Objects.requireNonNull(), added unit tests for several existing parameter checks, fixed small typo in CONTRIBUTING.md.

Signed-off-by: Heribert Hirth <git@frostcode.de>
A few changes to naming of context methods
Some changes around context
Remove dead code and add comment on daemons
correct spelling
Make ReadStream and WriteStream generic in type written/read
Reduce number of connections in test so test passes without having to tweak kernel parameters
Merge branch 'parameterchecks' of github.com:heri333/vert.x into heri333-parameterchecks
Merge branch 'heri333-parameterchecks'