Add support for delta bundles

Earlier a snapshot bundle would describe the full state of OPA's
policy/data and any update would require first erasing the state from
the existing bundle and then activating the new bundle.

This commit introduces a new bundle type called "delta".
Delta bundles contain patches to data instead of snapshots.
They allow users to efficiently make updates to OPA's data
cache.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>