Add conditional index to aid store deserialization.

[rtibbles]

Summary

• Adds conditional index and adds migration

Reviewer guidance

I tried to write a test, but a few attempts didn't yield anything much - and I assume the bigger test will be whether the index gets used on real world data.

Issues addressed

Fixes #228

[bjester]

Looks correct to me. If it doesn't work, the only thought would be that we should use less than and greater than against the partition field instead of LIKE (but I think postgres might do this internally)