ByteBufAllocator API w/ ByteBuf perf improvements

This commit introduces a new API for ByteBuf allocation which fixes
issue #643 along with refactoring of ByteBuf for simplicity and better
performance. (see #62)

A user can configure the ByteBufAllocator of a Channel via
ChannelOption.ALLOCATOR or ChannelConfig.get/setAllocator().  The
default allocator is currently UnpooledByteBufAllocator.HEAP_BY_DEFAULT.

To allocate a buffer, do not use Unpooled anymore. do the following:

  ctx.alloc().buffer(...); // allocator chooses the buffer type.
  ctx.alloc().heapBuffer(...);
  ctx.alloc().directBuffer(...);

To deallocate a buffer, use the unsafe free() operation:

  ((UnsafeByteBuf) buf).free();

The following is the list of the relevant changes:

- Add ChannelInboundHandler.freeInboundBuffer() and
  ChannelOutboundHandler.freeOutboundBuffer() to let a user free the
  buffer he or she allocated. ChannelHandler adapter classes implement
  is already, so most users won't need to call free() by themselves.
  freeIn/OutboundBuffer() methods are invoked when a Channel is closed
  and deregistered.

- All ByteBuf by contract must implement UnsafeByteBuf. To access an
  unsafe operation: ((UnsafeByteBuf) buf).internalNioBuffer()

- Replace WrappedByteBuf and ByteBuf.Unsafe with UnsafeByteBuf to
  simplify overall class hierarchy and to avoid unnecesary instantiation
  of Unsafe instances on an unsafe operation.

- Remove buffer reference counting which is confusing

- Instantiate SwappedByteBuf lazily to avoid instantiation cost

- Rename ChannelFutureFactory to ChannelPropertyAccess and move common
  methods between Channel and ChannelHandlerContext there. Also made it
  package-private to hide it from a user.

- Remove unused unsafe operations such as newBuffer()

- Add DetectionUtil.canFreeDirectBuffer() so that an allocator decides
  which buffer type to use safely
Reduce the chance of RejectedExecutionException

When a Netty application shuts down, a user often sees a REE
(RejectedExecutionException).

A REE is raised due to various reasons we don't have control over, such
as:

- A client connects to a server while the server is shutting down.

- An event is triggered for a closed Channel while its event loop is
  also shutting down.  Some of them are:
  - channelDeregistered (triggered after a channel is closed)
  - freeIn/OutboundBuffer (triggered after channelDeregistered)
  - userEventTriggered (triggered anytime)

To address this issue, a new method called confirmShutdown() has been
added to SingleThreadEventExecutor.  After a user calls shutdown(),
confirmShutdown() runs any remaining tasks in the task queue and ensures
no events are triggered for last 2 seconds.  If any task are added to
the task queue before 2 seconds passes, confirmShutdown() prevents the
event loop from terminating by returning false.

Now that SingleThreadEventExecutor needs to accept tasks even after
shutdown(), its execute() method only rejects the task after the event
loop is terminated (i.e. isTerminated() returns true.)  Except that,
there's no change in semantics.

SingleThreadEventExecutor also checks if its subclass called
confirmShutdown() in its run() implementation, so that Netty developers
can make sure they shut down their event loop impementation correctly.

It also fixes a bug in AioSocketChannel, revealed by delayed shutdown,
where an inboundBufferUpdated() event is triggered on a closed Channel
with deallocated buffers.

Caveats:

Because SingleThreadEventExecutor.takeTask() does not have a notion of
timeout, confirmShutdown() adds a dummy task (WAKEUP_TASK) to wake up
takeTask() immediately and instead sleeps hard-coded 100ms.  I'll
address this issue later by modifying takeTask() times out dynamically.

Miscellaneous changes:

SingleThreadEventExecutor.wakeup() now has the default implementation.
Instead of interrupting the current thread, it simply adds a dummy task
(WAKEUP_TASK) to the task queue, which is more elegant and efficient.
NioEventLoop is the only implementation that overrides it. All other
implementations' wakeup()s were removed thanks to this change.
Adding WebSocket Example using WebSocketServerProtocolHandler.
Add a line-based frame decoder with good performance.

Using DelimiterBasedFrameDecoder with Delimiters.lineDelimiter() has
quadratic performance in the size of the input buffer.  Needless to
say, the performance degrades pretty quickly as the size of the buffer
increases.  Larger MTUs or loopback connections can make it so bad that
it appears that the code is "busy waiting", when in fact it's spending
almost 100% of the CPU time in DelimiterBasedFrameDecoder.indexOf().

Add a new LineBasedFrameDecoder that decodes line-delimited frames
in O(n) instead of DelimiterBasedFrameDecoder's O(n^2) implementation.
In OpenTSDB's telnet-style protocol decoder this resulted in throughput
increases of an order of magnitude.

Change DelimiterBasedFrameDecoder to automatically detect when the
frames are delimited by line endings, and automatically switch to
using LineBasedFrameDecoder under the hood.  This means that all Netty
applications out there that using the combo DelimiterBasedFrameDecoder
with Delimiters.lineDelimiter() will automatically benefit from the
better performance of LineBasedFrameDecoder, without requiring a code
change.
Make CompatibleMarshallingDecoder discard the inbound buffer once TooLongFrameException is raised

(See #768)

Once too long object is received, CompatibleMarshallingDecoder has to
discard all input from now on, just like MarshallingDecoder does.
Otherwise, the decoder will raise more exceptions because the decoder
has no idea anymore where the object starts.

Before this fix, SerialThreadLocalCompatibleMarshallingDecoderTest
logged many additional exceptions raised by the decoder after test is
finished.
Remove unexpected bad text

Doh!
Add JVM options for faster test runs
Enable assertions only for Netty classes
Remove redundant exception message
Implement Bootstrap.toString() and use it in the testsuite

By implementing Bootstrap.toString() and printing it for each test case helps us figure out which transport is causing a trouble.
Ensure lazily instantiated outbound buffers are instantiated from the event loop
Add proper toString() implementation for internal ChannelFactory impls
fixing type on event/send argument