close[t:4574] merging 4574 to main. fixes the hcad deadlock found by Tim's stress test, which adds and drops indexes concurrent with queries and insertions. transactions no longer keep trollback nodes pinned after an operation, but instead always unpin them.

this merge also introduces a lot of improvements to our rollback code, in terms of clarity and consistency. to that end, variable names and function names were improved, as well as more documentation of the rollback logic in rollback.h and log-internal.h

roll.h is removed because it is a dead file.


git-svn-id: file:///svn/toku/tokudb@41576 c7de825b-a66e-492c-adef-691d508d4ae1