Add an operation that resets the state of HttpObjectDecoder

Motivation:

Currently, it is impossible to give a user the full control over what to do in response to the request with 'Expect: 100-continue' header.  Currently, a user have to do one of the following:

- Accept the request and respond with 100 Continue, or
- Send the reject response and close the connection.

.. which means it is impossible to send the reject response and keep the connection alive so that the client sends additional requests.

Modification:

Added a public method called 'reset()' to HttpObjectDecoder so that a user can reset the state of the decoder easily.  Once called, the decoder will assume the next input will be the beginning of a new request.

HttpObjectAggregator now calls `reset()`right after calling 'handleOversizedMessage()' so that the decoder can continue to decode the subsequent request even after the request with 'Expect: 100-continue' header is rejected.

Added relevant unit tests / Minor clean-up

Result:

This commit completes the fix of #2211