Small performance improvements

Modifications:

- Added a static modifier for `CompositeByteBuf.Component`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.
- Removed unnecessary boxing/unboxing operations in `HttpResponseDecoder`, `RtspResponseDecoder`, `PerMessageDeflateClientExtensionHandshaker` and `PerMessageDeflateServerExtensionHandshaker`.
A boxed primitive is created from a `String`, just to extract the unboxed primitive value.
- Removed unnecessary 3 times calculations in `DiskAttribute.addContent(...)`.
- Removed unnecessary checks if file exists before call `mkdirs()` in `NativeLibraryLoader` and `PlatformDependent`.
Because the method `mkdirs()` has this check inside.
- Removed unnecessary `instanceof AsciiString` check in `StompSubframeAggregator.contentLength(StompHeadersSubframe)` and `StompSubframeDecoder.getContentLength(StompHeaders, long)`.
Because `StompHeaders.get(CharSequence)` always returns `java.lang.String`.