crimson/seastore: add extent map tree

crimson/seastore: add extent map tree

  add extentmap b-tree implementation.

  extentmap root is not stored in extentmap manager. only stored in
onode leaf.

  The transactions under SeaStore is fully isolated with each other,
  and can be failed due to potential conflicts.

  If extentmap manager store the tree root, will cause the following
issues:
  1. The updated extmap_root with depth/laddr can be seen by other transactions,
     which is not valid before the transaction is submitted successfully;
  2. The updated extmap_root needs to be reverted if the transaction is failed;

  imbedded ceph_le64 in laddr_le_t, and provide operator= in laddr_le_t
  in case allow ceph_lexx to be key in fixed_kv_node_layout.h

Signed-off-by: chunmei-liu <chunmei.liu@intel.com>