Reuse previous created HttpHeaders by HttpObjectAggregator

Motivation:

HttpObjectAggregator currently creates a new FullHttpResponse / FullHttpRequest for each message it needs to aggregate. While doing so it also creates 2 DefaultHttpHeader instances (one for the headers and one for the trailing headers). This is bad for two reasons:
  - More objects are created then needed and also populate the headers is not for free
  - Headers may get validated even if the validation was disabled in the decoder

Modification:

- Wrap the previous created HttpResponse / HttpRequest and so reuse the original HttpHeaders
- Reuse the previous created trailing HttpHeader.
- Fix a bug where the trailing HttpHeader was incorrectly mixed in the headers.

Result:

- Less GC
- Faster HttpObjectAggregator implementation
[#2650] Allow to disable http header validation in SpdyHttpDecoder and SpdyHttpCodec

Motivation:

HTTP header validation can be expensive so we should allow to disable it like we do in HttpObjectDecoder.

Modification:

Add constructor argument to disable validation.

Result:
Performance improvement
[#2651] Fix possible infinite-loop when cancel tasks

Motivations:
In our new version of HWT we used some kind of lazy cancelation of timeouts by put them back in the queue and let them pick up on the next tick. This  multiple problems:
 - we may corrupt the MpscLinkedQueue if the task is used as tombstone
 - this sometimes lead to an uncessary delay especially when someone did executed some "heavy" logic in the TimeTask

Modifications:
Use a Lock per HashedWheelBucket for save and fast removal.

Modifications:
Cancellation of tasks can be done fast and so stuff can be GC'ed and no more infinite-loop possible