Lock tree further disentangled from transactions.
Now uses a function to compare txnids all the time instead of direct
integer compare, also removed asserts checking for them to be non 0,
since 0 is a valid TXN id.

git-svn-id: file:///svn/tokudb@3256 c7de825b-a66e-492c-adef-691d508d4ae1