Addresses #2155 refs[t:2155] Renamed variables,

Added missing state transaction (turn around on checkpoint begin with no transactions)
Added error checking for unlink (anything but 0/ENOENT is real error)

Changes due to code review with Barry

git-svn-id: file:///svn/toku/tokudb@15848 c7de825b-a66e-492c-adef-691d508d4ae1