Introduce MessageAggregator and DecoderResultProvider

Motivation:

We have different message aggregator implementations for different
protocols, but they are very similar with each other.  They all stems
from HttpObjectAggregator.  If we provide an abstract class that provide
generic message aggregation functionality, we will remove their code
duplication.

Modifications:

- Add MessageAggregator which provides generic message aggregation
- Reimplement all existing aggregators using MessageAggregator
- Add DecoderResultProvider interface and extend it wherever possible so
  that MessageAggregator respects the state of the decoded message

Result:

Less code duplication