Fix example for Http Upload

Motivation:
The example mis handle two elements:
1) Last message is a LastHttpContent and is not taken into account by
the server handler
2) The client makes a sync on last write (chunked) but there is no flush
before, therefore the sync is waiting forever.

Modifications:
1) Take into account the message LastHttpContent in simple Get.
2) Removes sync but add flush for each post and multipost parts

Results:
Example is no more blocked after get test.

Should be done also in 4.0 and Master (similar changes)
The below line is waiting for the future closure, right?

    channel.closeFuture().sync();

Is not that enough?
If we change write to writeAndFlush, we should do this for "all" write, since there is a condition (<code>isChunked()</code> guards this write), and as there is no way to wait on the <code>channel.flush()</code> that i below (is it?).