Tests were revised to no longer expect rewind() to encounter an error and resume. In most tests, rewind() is a NOP because the change stream has no events to return upon creation.

Since Watch now always executes aggregate commands with APM, the check for startAtOperationTime support was moved to createResumeCallable(). We continue to only capture operationTime from the very first aggregate command.

Various assertions for current() returning null were also changed to instead check valid(). Both forms are equivalent, but checking valid() is more consistent with our iteration examples.