Changed Http2ClientConnectionHandler to print out the aggregated buffers content

Motivation:
When running the Http2Client the data returned from the server, the
"Hello World" string, is supposed to be printed but instead the following
is displayed:
Received message:

Modifications:
Use the aggregated buffer to print out.

Result:
The example now logs the correct data sent from the server:
Received message: Hello World
Fix the build timeout when 'leak' profile is active

Motivation:

AbstractByteBufTest.testInternalBuffer() uses writeByte() operations to
populate the sample data.  Usually, this isn't a problem, but it starts
to take a lot of time when the resource leak detection level gets
higher.

In our CI machine, testInternalBuffer() takes more than 30 minutes,
causing the build timeout when the 'leak' profile is active (paranoid
level resource detection.)

Modification:

Populate the sample data using ThreadLocalRandom.nextBytes() instead of
using millions of writeByte() operations.

Result:

Test runs much faster when leak detection level is high.
Let OkResponseHandler extend SimpleChannelInboundHandler

Motivation:

OkResponseHandler is the last handler in the pipeline of the HTTP CORS
example.  It is responsible for releasing all messages it handled.

Modification:

Extend SimpleChannelInboundHandler instead of ChannelHandlerAdapter

Result:

Fixed a leak
Split a JSON byte stream into JSON objects/arrays. Fixes #2536

Motivation:

See GitHub Issue #2536.

Modifications:

Introduce the class JsonObjectDecoder to split a JSON byte stream
into individual JSON objets/arrays.

Result:

A Netty application can now handle a byte stream where multiple JSON
documents follow eachother as opposed to only a single JSON document
per request.
Fix a buffer leak in JsonObjectDecoderTest
Fix an inspector warning in JsonObjectDecoder
[#2618] Introduce ChannelPromise.unvoid() and ChannelFuture.isVoid()

Motivation:

There is no way for a ChannelHandler to check if the passed in ChannelPromise for a write(...) call is a VoidChannelPromise. This is a problem as some handlers need to add listeners to the ChannelPromise which is not possible in the case of a VoidChannelPromise.

Modification:

- Introduce ChannelFuture.isVoid() which will return true if it is not possible to add listeners or wait on the result.
- Add ChannelPromise.unvoid() which allows to create a ChannelFuture out of a void ChannelFuture which supports all the operations.

Result:

It's now easy to write ChannelHandler implementations which also works when a void ChannelPromise is used.