Fix derp in commit protocol

I took over the old guard()->tick() from the old commit protocol
w/o noticing that it was NOT in fact a read of the true global
tick. Thank you invariants for disabusing me.

Now tests pass with invariants on.
txn_btree: equip with rsearch helpers
txn_btree: remove commented out txn_proto1 test cases
Merge pull request #12 from kohler/master

Fix derp in commit protocol
txn_btree: add some rsearch test case
Merge remote-tracking branch 'github/master' into mbscan