Make sure HttpVersion checks if the version string is not empty

.. which was the behavior in 3.x.
SPDY: code cleanup
Merge pull request #1928 from jpinner/spdy_cleanup_master

SPDY: code cleanup
[#1934] Correctly log handshake errors and not print them to STDERR
More robust pipeline manipulation while upgrading to WebSocket
- This commit allows a user to write its first web socket frame right after calling WebSocketServerHandshaker.handshake() rather than adding a listener to the future it returns.
- Should fix #1933
[#1933] Correctly add the wsencoder before the httpencoder as the httpencoder also handle ByteBuf
[#1936] Fix example in javadoc
[#1943] Unpooled.copiedBuffer(ByteBuf pooled) should always return unpooled ByteBuf
[#1812] Allow for inline for most common cases when use NioByteUnsafe.read()
[#1812] Rework ByteToMessageDecoder.channelRead(..) method to allow for inlining
Revert "[#1812] Rework ByteToMessageDecoder.channelRead(..) method to allow for inlining"

This reverts commit b7262ab3da124f4d2db297e33c117ebff6cf1964.
[#1812] Rework ByteToMessageDecoder.channelRead(..) method to allow for inlining
Make all mutable fields in WebSocketClientHandshaker volatile

- Fixes #1945
[#1947] Handle RejectExecutionException graceful for outbound operations
Replace UniqueName with Constant and ConstantPool

- Proposed fix for #1824

UniqueName and its subtypes do not allow getting the previously registered instance.  For example, let's assume that a user is running his/her application in an OSGi container with Netty bundles and his server bundle.  Whenever the server bundle is reloaded, the server will try to create a new AttributeKey instance with the same name.  However, Netty bundles were not reloaded at all, so AttributeKey will complain that the name is taken already (by the previously loaded bundle.)

To fix this problem:

- Replaced UniqueName with Constant, AbstractConstant, and ConstantPool.  Better name and better design.

- Sctp/Udt/RxtxChannelOption is not a ChannelOption anymore.  They are just constant providers and ChannelOption is final now.  It's because caching anything that's from outside of netty-transport will lead to ClassCastException on reload, because ChannelOption's constant pool will keep all option objects for reuse.

- Signal implements Constant because we can't ensure its uniqueness anymore by relying on the exception raised by UniqueName's constructor.
Add a shortcut method for collision-free naming
[#1812] All to have NioMessageUnsafe.read() inlined