PackConfig: add entry for minimum size to index

PackConfig: add entry for minimum size to index

The object size index can have up to #(blobs-in-repo) entries, taking
a relevant amount of memory. Let operators configure the threshold size
to include objects in the size index.

The index will include objects with size *at or above* this
value (with -1 for none). This is more effective for the
filter-by-size case.

Lowering the threshold adds more objects to the index. This improves
performance at the cost of memory/storage space. For the object-size
case, more calls will use the index instead of reading IO. For the
filter-by-size case, lower threshold means better granularity (if
ObjectReader#isSmallerThan is implemented based only on the index).

Change-Id: I6ccd9334adbbc2abf95fde51dbbfc85b8230ade0