R1:7aa12f192a6b

Add PhabricatorQueryIterator, for buffered iteration over a CursorPagedPolicyAwareQuery

Summary:
See D19446. This should make it easier to process larger, more complex result sets in constant memory.

Today, `LiskMigrationIterator` takes constant memory but can't apply `needX()` reqeusts or `withY(...)` constraints.

Using a raw `Query` can handle this stuff, but requires memory proportional to the size of the result set.

Offer the best of both worlds: constant memory and full access to the power of…