Small performance improvements

Modifications:

- Added a static modifier for CompositeByteBuf.Component.
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.
A boxed primitive is created from a String, just to extract the unboxed primitive value.
- Removed unnecessary checks if file exists before call mkdirs() in NativeLibraryLoader and PlatformDependent.
Because the method mkdirs() has this check inside.

Conflicts:
	codec-http/src/main/java/io/netty/handler/codec/http/multipart/DiskAttribute.java
	codec-stomp/src/main/java/io/netty/handler/codec/stomp/StompSubframeAggregator.java
	codec-stomp/src/main/java/io/netty/handler/codec/stomp/StompSubframeDecoder.java