Fix a stall write in EpollSocketChannel

Motivation:

When a ChannelOutboundBuffer contains a series of entries whose messages
are all empty buffers, EpollSocketChannel sometimes fails to remove
them. As a result, the result of the write(EmptyByteBuf) is never
notified, making the user application hang.

Modifications:

- Add ChannelOutboundBuffer.removeBytes(long) method that updates the
  progress of the entries and removes them as much as the specified
  number of written bytes.  It also updates the reader index of
  partially flushed buffer.
  - Make both NioSocketChannel and EpollSocketChannel use it to reduce
    code duplication
  - Replace EpollSocketChannel.updateOutboundBuffer()
- Refactor EpollSocketChannel.doWrite() for simplicity
  - Split doWrite() into doWriteSingle() and doWriteMultiple()
- Do not add a zero-length buffer to IovArray
- Do not perform any real I/O when the size of IovArray is 0

Result:

Another regression is gone.