Clean up MpscLinkedQueue, fix its leak, and make it work without Unsafe

Motivation:

MpscLinkedQueue has various issues:
- It does not work without sun.misc.Unsafe.
- Some field names are confusing.
  - Node.tail does not refer to the tail node really.
  - The tail node is the starting point of iteration. I think the tail
    node should be the head node and vice versa to reduce confusion.
- Some important methods are not implemented (e.g. iterator())
- Not serializable
- Potential false cache sharing problem due to lack of padding
- MpscLinkedQueue extends AtomicReference and thus exposes various
  operations that mutates the internal state of the queue directly.

Modifications:

- Use AtomicReferenceFieldUpdater wherever possible so that we do not
  use Unsafe directly. (e.g. use lazySet() instead of putOrderedObject)
- Extend AbstractQueue to implement most operations
- Implement serialization and iterator()
- Rename tail to head and head to tail to reduce confusion.
- Rename Node.tail to Node.next.
- Fix a leak where the references in the removed head are not cleared
  properly.
- Add Node.clearMaybe() method so that the value of the new head node
  is cleared if possible.
- Add some comments for my own educational purposes
- Add padding to the head node
  - Add FullyPaddedReference and RightPaddedReference for future reuse
- Make MpscLinkedQueue package-local so that a user cannot access the
  dangerous yet public operations exposed by the superclass.
  - MpscLinkedQueue.Node becomes MpscLinkedQueueNode, a top level class

Result:

- It's more like a drop-in replacement of ConcurrentLinkedQueue for the
  MPSC case.
- Works without sun.misc.Unsafe
- Code potentially easier to understand
- Fixed leak (related: #2372)