db: runtime_events table: add indexes

[mitjat]

Based on a report from @lukaw3d where the following two queries timed out:

• https://index-staging.oasislabs.com/v1/emerald/events?tx_hash=1ada4eb14e50d7c5426f707b6d636e2222280308f09356c1cb2c14503fe927e3
• https://index-staging.oasislabs.com/v1/emerald/events?limit=1

This PR adds an index for each of those two queries.

Already applied in staging and prod DBs. Queries are fast now.

[pro-wh]

can we add primary key (runtime, round, tx_index) instead of the _round index?

edit: oh can't, because multiple events \:

[Andrew7234]

Lgtm!