Limits in oplog code

- distinguish queries with limits and sorts
- separate the concept of matching set from published set
- keep a separate buffer for N documents outside of limit to reduce mongo
  lookups
Always use limit to separate ordered code vs unordered
Export sorter in Minimongo namespace
Fix Dummy Structure

actually Dummy-structure was supposed to be so dummy, it would be trivially easy
to write a bug-free 20-line code for it. But I failed. To dummy structure...
Correctly distinguish queries with limits and sorts
Fix typos
Typo + always use inequalities with C-style comparators
A simple test for limits

and it does pass!
More tests on updates. Clarifying comment