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
Hmm, looking on the web, it seems that:
- both resturns the same "value"
- but parseInt produced a primitive int
- and valueOf returns an Integer (in fact from Integer.valueOf(parseInt(val, 10)))
So depending on the needs, one should use the first or the second.
Since here the HttpResponseStatus is waiting for an "int", the parseInt should be faster since the second will imply to create an Integer and then to get the primitive int instead...

So parseInt should be faster here, am I right?