[t:3982] #3982 Fix some broken upgrade mechanisms.  Fix misnamed field in ydb.c.  Replace brt header field num_blocks_to_upgrade with explicit num_blocks_to_upgrade_13 and num_blocks_to_upgrade_14.  Add some TODO comments.

git-svn-id: file:///svn/toku/tokudb@35836 c7de825b-a66e-492c-adef-691d508d4ae1