Make pipeline handler behave better when removed

[Lukasa]

Motivation:

The HTTPServerPipelineHandler is removed from pipelines in many cases,
but the most common case is over upgrade. While the handler is
removable, it doesn't make any effort to ensure that it leaves the
pipeline in a sensible state, which is pretty awkward.

In particular, there are 3 things the pipeline handler may be holding
on to that can lead to damage. The first is pipelined requests: if there
are any, they should be delivered, as the user may be deliberately
allowing pipelining.

The second thing is read() calls. The HTTPServerPipelineHandler exerts
backpressure on clients that aggressively pipeline by refusing to read
from the socket. If that happens, and then the handler is removed from
the channel, it will "forget" to restart reading from the socket on the
way out. That leaves the channel quietly in a state where no reads will
occur ever again, which is pretty uncool.

The third thing is quiescing. The HTTPServerPipelineHandler catches
quiescing events and allows them to deliver a response before closing a
connection. If that has happened when the pipeline handler is removed,
it should fall back to the behaviour as though it were not there.

Modifications:

• Added a handlerRemoved implementation to play event state that should
  be replayed.
• Added a channelInactive implementation to drop data.

Result:

More graceful handler removal.

[weissi]

Thank you, lgtm!