ObjectReader: New #isNotLargerThan method

ObjectReader: New #isNotLargerThan method

Partial clones filter the objects to send by size calling
ObjectReader#getObjectSize per object. This method reads the object from
storage to get the size, which can be expensive.

Offer a #isNotLargerThan method. The default implementation reads the
object, but subclasses can override it with more efficient lookups (e.g.
adding an index).

isNotLargerThan gives implementors more options to optimize than
getObjectIndex (e.g. can be implemented storing only object over certain
size).

Change-Id: Iefd4b1370cb9144f15cc0391286aeeb365e6ea87