Move core data structure of DetachedBuffer into BufferNode

1/ Supports mutiple DetachedBuffer(s) to be created from a single
BufferNode.
2/ Separate core data structure and logic (i.e. BufferNode) out of the
IPC layer (i.e. DetachedBufferChannel).
3/ DetachedBufferChannel will be renamed to BufferChannel in future
and becomes the universal backend of GraphicBuffer. Thus we are naming
the new class BufferNode instead of DetachedBufferNode.
4/ Move BufferHubDefs out of bufferhub_rpc.h, as BufferHubDefs is not
directly related to the PDX IPCm echanism.

Bug: 112011098
Bug: 112012812
Bug: 111976433
Bug: 70046255
Test: buffer_hub-test
Change-Id: Ieba19329a0e823c78f21611462dedacdad3aab85