Auto-generate the handler name when null is specified as a name

Motivation:

There's no way to generate the name of a handler being newly added
automatically and reliably.

For example, let's say you have a routine that adds a set of handlers to
a pipeline using addBefore() or addAfter().  Because addBefore() and
addAfter() always require non-conflicting non-null handler name, making
the multiple invocation of the routine on the same pipeline is
non-trivial.

Modifications:

- If a user specifies null as the name of the new handler,
  DefaultChannelPipeline generates one.
- Update the documentation of ChannelPipeline to match the new behavior

Result:

A user doesn't need to worry about name conflicts anymore.