Resurrect channel deregistration and constructor changes

Motivation:

Due to the complexity of handling deregistration and re-registration of
a channel, we previously decided to remove the deregister() operation
completely to simplify our code.  However, we realized that it shouldn't
be that complicated to implement it during our discussion about making
I/O scheduling more flexible and more customizable [1], and thus the
removal of deregistration and re-registration is unnecessary now.

Modification:

- Revert commit c149f4bcc0c0d02aa1abcd5e39c155a9e598822e
- Revert commit e743a27e752f0badaf71688457793d0aa365a3ac
- Make some additional adjustments

Result:

- deregister(), fireChannelUnregistered(), and channelRegistered() were
  added back..
- Channel constructors do not require an EventLoop anymore.

[1] https://github.com/netty/netty/issues/2250