Rework Payload API and Behavior

[OlegDokuka]

Payload API has inconsistent and absolutely confusing behavior:

##Expected
case 1

var p = ByteBufPayload.create(data, Unpooled.EMPTY)
assertTrue(p.hasMetadata())
assertEqueals(p.metadata(), Unpooled.EMPTY) 

Unexpected

case 2

var p = ByteBufPayload.create(data, null)
assertFalse(p.hasMetadata())
assertNull(p.metadata()) // fails because Payload always returns Unpooled.EMPTY

[OlegDokuka]

related #771