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
wip test
Fix the test on limit-requery ; keep docs in _published and _unpublishedBuffer consistently w/o _id
wip fixing the appends to buffer
wip changed published document can go into buffer
Fix tests for oplog
Fix typo: addPublished -> addBuffered;

Revert the population of initialQuerying back to _addMatching everything.
Tests behave correctly in both oplog and non-oplog mode
Tests on newly supported cursor types
Remove Sorter class from MinimongoTest as it is already exported in Minimongo symbol
More comments, explicity, fixed ambiguities
More self-checks with throws (asserts)