HTTP/2 Inbound Flow Control Connection Window Issues

Motivation:
The inbound flow control code was returning too many bytes to the connection window.  This was resulting in GO_AWAYs being generated by peers with the error code indicating a flow control issue.  Bytes were being returned to the connection window before the call to returnProcessedBytes. All of the state representing the connection window was not updated when a local settings event occurred.

Modifications:
The DefaultHttp2InboundFlowController will be updated to correct the above defects.
The unit tests will be updated to reflect the changes.

Result:
Inbound flow control algorithm does not cause peers to send flow control errors for the above mentioned cases.
Fix a race condition where handler is removed before unregistration

Related: #3156

Motivation:

Let's say we have a channel with the following pipeline configuration:

  HEAD --> [E1] H1 --> [E2] H2 --> TAIL

when the channel is deregistered, the channelUnregistered() methods of
H1 and H2 will be invoked from the executor thread of E1 and E2
respectively. To ensure that the channelUnregistered() methods are
invoked from the correct thread, new one-time tasks will be created
accordingly and be scheduled via Executor.execute(Runnable).

As soon as the one-time tasks are scheduled,
DefaultChannelPipeline.fireChannelUnregistered() will start to remove
all handlers from the pipeline via teardownAll(). This process is
performed in reversed order of event propagation. i.e. H2 is removed
first, and then H1 is removed.

If the channelUnregistered() event has been passed to H2 before H2 is
removed, a user does not see any problem.

If H2 has been removed before channelUnregistered() event is passed to
H2, a user will often see the following confusing warning message:

  An exceptionCaught() event was fired, and it reached at the tail of
  the pipeline. It usually means the last handler in the pipeline did
  not handle the exception.

Modifications:

To ensure that the handlers are removed *after* all events are
propagated, traverse the pipeline in ascending order before performing
the actual removal.

Result:

A user does not get the confusing warning message anymore.
example: memcache: fix set command

Motivation:

The example MemcacheClient set command doesn't work.

Modifications:

Fill the extras field buffer with zeros so that it gets written to the
request payload.

Result:

The example MemcacheClient set command works.
Lazily check for root, avoids unnecessary errors & resources

Motivation:

io.netty.util.internal.PlatformDependent.isRoot() depends on the IS_ROOT field which is filled in during class initialization. This spawns processes and consumes resources, which are not generally necessary to the complete functioning of that class.

Modifications:

This switches the class to use lazy initialization this field inside of the isRoot() method using double-checked locking (http://en.wikipedia.org/wiki/Double-checked_locking).

Result:

The first call to isRoot() will be slightly slower, at a tradeoff that class loading is faster, uses fewer resources and platform errors are avoided unless necessary.