More readable code, less arguments

[magicprinc]

• HandlerRegistration has two very similar dispatch methods: very confusing.
• param handler is used only in 50% cases (MessageConsumerImpl, but not ReplyHandler)
• In case of MessageConsumerImpl param handler is the same as MessageConsumerImpl.handler and is only used to protected it from consumer.unregister() during dispatch.
• handlerMap.compute can be used instead of merge producing more compact code and without unnecessary (looser) object creation
• more comments, better field/variable names, better exception messages

I have made several changes to make the code more compact, readable and understandable.
Heavily tested!

PR:
#4850

(from https://github.com/magicprinc/vert.x/commits/bugfix/compact_args)

Includes Issue #4836 with PR #4847

[magicprinc]

Updated version as PR: #4957