[t:4778] upgrade subtree estimates to stat64info using real code. fix #4778

refactor existing node deserialization code so that upgrading a header can
call deserialization code on the root node and get stat64info instead of
deserializing it manually.


git-svn-id: file:///svn/toku/tokudb@43459 c7de825b-a66e-492c-adef-691d508d4ae1