store/postgres: Add an index on event_meta_data(source)

[lutter]

We find event_meta_data during block reversions by the source attribute,
and not having an index on it slows down those queries, and therefore block
reversion, dramatically. In one case, query time for a block reversion goes
from 18s to 5ms after adding the index.

[lutter]

Building this index in production will take about 3 minutes.

[lutter]

I actually just added the index manually to production so that rolling this out won't slow down startup