Fixes for compression codecs

Motivation:

Fixed founded mistakes in compression codecs.

Modifications:

- Changed return type of ZlibUtil.inflaterException() from CompressionException to DecompressionException
- Updated @throws in javadoc of JZlibDecoder to throw DecompressionException instead of CompressionException
- Fixed JdkZlibDecoder to throw DecompressionException instead of CompressionException
- Removed unnecessary empty lines in JdkZlibEncoder and JZlibEncoder
- Removed public modifier from Snappy class
- Added MAX_UNCOMPRESSED_DATA_SIZE constant in SnappyFramedDecoder
- Used in.readableBytes() instead of (in.writerIndex() - in.readerIndex()) in SnappyFramedDecoder
- Added private modifier for enum ChunkType in SnappyFramedDecoder

Result:

Fixed sum overflow in Bzip2HuffmanAllocator, improved exceptions in ZlibDecoder implementations, hid Snappy class

Conflicts:
	codec/src/main/java/io/netty/handler/codec/compression/Bzip2HuffmanAllocator.java