crimson/os/seastore/lba_manager: replace btree implementation

crimson/os/seastore/lba_manager: replace btree implementation

Replace previous implementation with one based around an internal
iterator interface.  Besides simplifying the implementation and
removing duplicate lookups in the allocation pathway, this implementation
should correct a design problem in the prior implementation wherein
LBALeafNode::find_hole couldn't see the first element of the subsequent
node and therefore assumed that there was one at get_meta().end.

This patch removes the btree logic from lba_btree_node_impl.* leaving
the LBAInternalNode and LBALeafNode layout in lba_btree_node.*.
lba_btree.h/cc now have the main btree update/query logic.

Signed-off-by: Samuel Just <sjust@redhat.com>