R1:ab92242e00b3

Extend PhabricatorPolicyQuery from PhabricatorOffsetPagedQuery

Summary:
A few goals here:

- Slightly simplify the Query classtree -- it's now linear: `Query` -> `OffsetPagedQuery` (adds offset/limit) -> `PolicyQuery` (adds policy filtering) -> `CursorPagedPolicyQuery` (adds cursors).
- Allow us to move from non-policy queries to policy queries without any backward compatibility breaks, e.g. Conduit methods which accept 'offset'.
- Separate the client limit ("limit") from the datafetch hint limit…