storage/metamorphic: run scans on batches as well

[jbowens]

The metamorphic test did not previously run scan operations against readers other than the Engine due to a limitation of RocksDB. This commit addresses the comment, performing scans against batches and engine state alike.

Epic: none Release note: none

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[jbowens]

TFTR!

bors r=RaduBerinde

[craig[bot]]

Build succeeded:

• lint
• docker_image_amd64
• check_generated_code
• local_roachtest
• unit_tests
• examples_orms
• local_roachtest_fips
• acceptance
• linux_amd64_fips_build
• linux_amd64_build