Optimise response event listener

[mikkokar]

Problem

ResponseEventListener adds unnecessary pressure on heap:

• It constructs a StateMachine for the response that is being monitored. The underlying StateMachine loads of Map.Entry and Key objects.
• It uses a separate LiveHttpResponse builder for each ContentError, ContentEnd and ContentCancelled events.

Fix

• Replace StateMachine object with nested switch/case and if/else statements.
• Collapse ContentError, ContentEnd, and ContentCancelled event response body content callbacks in a single LiveHttpResponse builder to avoid excessive HTTP header copying.

Impact

On routing object based application router:

• HeaderEntry: ~ 40% less objects
• HeaderEntry[]: ~ 33% less objects
• Object[]: ~ 37% less objects
  And so on.

[dvlato]

Should we deal with this TODO / remove it? What do you think of dating TODO comments?

[mikkokar]

Hi David. This is an old TODO that appears in added lines because the code has been refactored.

A date could be helpful for code reviewers to spot old TODO items that have merely moved around. But I'm not opinionated about this.