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)