Fix the dead lock while failing SSL pending writes on a closed channel

Related issue: #2093

Motivation:

When a channel with SslHandler is closed with pending writes (either
unencrypted or encrypted), SslHandler.channelClosed() attempts to
acquire a lock and fail the futures of the pending writes.

If a user added a listener to any of the failed futures and the listener
attempts to write something, it will make SslHandler.handleDownstream()
invoked, which also attempts to acquire the same lock.

Because the lock is non-reentrant, even if these two lock acquisitions
are taking place in the same thread, the second attempt will block for
ever.

Modification:

Do not fail the futures while holding a lock.  Notify them after
releasing the lock.

Result:

One less dead lock