Add index on quorum_proposals.leaf_hash (#2208)

We look up by this column during state reconstruction, so we should
probably index it. The unique index will also prevent us from inserting
quorum proposals with different views but the same hash, which should not
be possible and would be a logic error if it happened.

sequencer/api/migrations/V42__index_quorum_proposal_leaf_hash.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX quorum_proposals_leaf_hash_idx ON quorum_proposals (leaf_hash);