Snapshot scans should only refuse scans with timestamps whose value is > now+error

We currently reject a snapshot scan timestamp if it's value if beyond clock->Now(). We should only reject it if it's value is beyond clock->Now() + error, since all values < clock->Now() + error can still be generated by perfectly valid servers.

We should wait for the timestamp to be safe in all cases.

Marking this as best effort as this does not make kudu return wrong values, it just makes it a little less tolerant to skew than it could be.