Unserialize on the fly to Save memory in Change.

This causes the 'info' field in ChangeRow objects to remain
a serialized string internally, and only be unserialized on the fly
when accessed. This avoids memory exhaustion in situations where
a lot of change objects need to be processed.

Change-Id: I4bc934035b3bc7b782339a89bcba54c2ea09adf8
Merge "Unserialize on the fly to Save memory in Change."