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 ("rawresultlimit") so we can make the heurstic smarter in the future if we want. Some discussion inline.

Test Plan: Expanded unit tests to cover offset behaviors.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3192