objectcache: add object segmentation support to BagOStuff

objectcache: add object segmentation support to BagOStuff

Use it for ApiStashEdit so that large PaserOutput can be stored.

Add flag to allow for value segmentation on set() in BagOStuff.
Also add a flag for immediate deletion of segments on delete().

BagOStuff now has base serialize()/unserialize() methods.

Bug: T204742
Change-Id: I0667a02612526d8ddfd91d5de48b6faa78bd1ab5