Trigger channelWritabilityChanged() later to avoid reentrance

Related: #3212

Motivation:

When SslHandler and ChunkedWriteHandler exists in a pipeline together,
it is possible that ChunkedWriteHandler.channelWritabilityChanged()
invokes SslHandler.flush() and vice versa. Because they can feed each
other (i.e. ChunkedWriteHandler.channelWritabilityChanged() ->
SslHandler.flush() -> ChunkedWriteHandler.channelWritabilityChanged() ->
..), they can fall into an inconsistent state due to reentrance (e.g.
bad MAC record at the remote peer due to incorrect ordering.)

Modifications:

- Trigger channelWritabilityChanged() using EventLoop.execute() when
  there's a chance where channelWritabilityChanged() can cause a
  reentrance issue
- Fix test failures caused by the modification

Result:

Fix the handler reentrance issues related with a
channelWritabilityChanged() event
Add SslHandler.renegotiate()

Related: #3125

Motivation:

We did not expose a way to initiate TLS renegotiation and to get
notified when the renegotiation is done.

Modifications:

- Add SslHandler.renegotiate() so that a user can initiate TLS
  renegotiation and get the future that's notified on completion
- Make SslHandler.handshakeFuture() return the future for the most
  recent handshake so that a user can get the future of the last
  renegotiation
- Add the test for renegotiation to SocketSslEchoTest

Result:

Both client-initiated and server-initiated renegotiations are now
supported properly.
HTTP/2 Draft 16

Motivation:
HTTP/2 draft 16 has been released https://tools.ietf.org/html/draft-ietf-httpbis-http2-16.

Modifications:
The HTTP/2 codec should be updated to support draft 16.

Result:
HTTP/2 codec is draft 16 compliant.