MongoDB has an unfixed bug in all supported versions 2.6-3.4 which
results in suboptimal index usage for `$or` queries when the query has
implicit `$and`s at the query root.
When adding `_rperm` to `$or` queries, Parse accidentally creates
queries which hit this bug.
The fix in this commit applies the suggested workaround of putting the
`_rperm` property inside all `$or` subdocuments, moving it from the top
level and leaving `$or` as the only top-level operator.
MongoDB Bug Link: https://jira.mongodb.org/browse/SERVER-13732
96a9588a6e