expand an oplog/limit test a little

- put more info in state comments
- add a few more assertions
- stop requiring "usesOplog &&" before some helpers
failing (with oplog) test for updating a sort key
oplog-limit: don't ignore modifiers to sort key

Previously we would ignore modifier updates to documents outside our
current buffer if they did not affect the selector but did affect the
sort key.
Merge branch 'limit-sort-overlap' into devel