Small performance improvements

Motivation:

Found performance issues via FindBugs.

Modifications:

- Removed unnecessary boxing/unboxing operations in `DefaultTextHeaders.convertToInt(CharSequence)` and `DefaultTextHeaders.convertToLong(CharSequence)`. A boxed primitive is created from a string, just to extract the unboxed primitive value.
- Added a static modifier for `DefaultHttp2Connection.ParentChangedEvent` class. This class is an inner class, but does not use its embedded reference to the object which created it. This reference makes the instances of the class larger, and may keep the reference to the creator object alive longer than necessary.

Result:

Performance improvements.