crimson: add node implementation for onode tree

crimson: add node implementation for onode tree

* node_t and friends are representations of in-mem and on-disk onode
  block/page/node. it might have different names in different contexts.
  but normally, they are the same thing and are loaded as a logical
  cached extent.
* delta_t holds the mutations against a given node onode. they are
  applied to memory once they are committed to disk as part of the
  log record.

regarding to the name of "simple-fltree", it is a short for simple
Flexible Layout Fixed Tree, which was designed by Samuel Just. My
implementation is a naive implementation of this design. But it
tries to be extensive and should be able to adapt to different policies
for rebalancing nodes when overflow/underflow. But there could be
other variant of flexible layout tree implementations in future,
so i added "simple" prefix.

Signed-off-by: Kefu Chai <kchai@redhat.com>