[PoC] Push simple filter conditions into $lookup stage.

[WaVEV]

In this PR we tackle the slow join pushing the simple conditions inside the $lookup. the condition is also fix a bug.

New query:

[{'$lookup': {'as': 'm2m_regress_selfrefer_references',
              'from': 'm2m_regress_selfrefer_references',
              'let': {'parent__field__0': '$_id'},
              'pipeline': [{'$match': {'$expr': {'$and': [{'$eq': ['$$parent__field__0',
                                                                   '$to_selfrefer_id']},
                                                          {'$eq': ['$from_selfrefer_id',
                                                                   ObjectId('687ad601daccf7b757ff46ec')]}]}}}]}},
 {'$unwind': '$m2m_regress_selfrefer_references'},
 {'$match': {'$expr': {'$eq': ['$m2m_regress_selfrefer_references.from_selfrefer_id',
                               ObjectId('687ad601daccf7b757ff46ec')]}}}]

Previous query

[{'$lookup': {'as': 'm2m_regress_selfrefer_references',
              'from': 'm2m_regress_selfrefer_references',
              'let': {'parent__field__0': '$_id'},
              'pipeline': [{'$match': {'$expr': {'$and': [{'$eq': ['$$parent__field__0',
                                                                   '$to_selfrefer_id']}]}}}]}},
 {'$unwind': '$m2m_regress_selfrefer_references'},
 {'$match': {'$expr': {'$eq': ['$m2m_regress_selfrefer_references.from_selfrefer_id',
                               ObjectId('687ad676396a5de149f2ea4d')]}}}]

[timgraham]

Does it address #309? Do we need some additional tests for other cases besides the changed test?

I'll have to take it on faith that this does the right thing, because it would take me a lot of time to understand this code. Some comments explaining variable names like promote_filters, pushed_expressions, and rerooted_replacement as well as what cases this covers (what are "simple cases"?) would be helpful.