[t:4963] fix another issue with reference counting in lock trees. we should not store a lock tree object in the manager if its reference count is zero, too many cases to deal with toku_ltm_get_lt. instead, atomically check if a lt's ref is 1 and remove it from the manager, so no race can occur between removing and adding another reference.


git-svn-id: file:///svn/toku/tokudb@44661 c7de825b-a66e-492c-adef-691d508d4ae1