Small performance improvements

Motivation:

Found performance issues via FindBugs and PMD.

Modifications:

- Removed unnecessary boxing/unboxing operations in `DefaultTextHeaders` and `HttpContentCompressor`. A boxed primitive is created from a string, just to extract the unboxed primitive value.
- Optimized static imports in `DefaultBinaryHeaders` and `DefaultTextHeaders`.
- 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.
- Added a static compiled `Pattern` to avoid compile it each time it is used when we need to replace some part of authority.
- Improved using of StringBuilders.

Result:

Performance improvements.
@idelpivnitskiy - Should we now check if `uri.isEmpty()` up front with the string changes you have made?