Added versioning to descriptor.
Changing a descriptor REQUIRES the version to increase.
Version 0 is reserved for a non-descriptor db. (cannot be set).
Not yet added to brtnodes.
Still possible to change descriptor with an open brt (which would not be upgraded).


git-svn-id: file:///svn/toku/tokudb@11198 c7de825b-a66e-492c-adef-691d508d4ae1