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
@trustin this was a bug before which is also fixed in this commit.