Don't err on duplicate slot decryption signatures

We might receive duplicate slot decryption signatures (e.g. if a key
shares message is delivered multiple times, or if we receive a keys
message if we've already seen a few of the corresponding shares
messages). Don't log and error if we try to insert these signatures into
the database.

Author: jannikluhn

rolling-shutter/keyperimpl/gnosis/database/gnosiskeyper.sqlc.gen.go

@@ -75,7 +75,8 @@ WHERE eon = $1;
 
 -- name: InsertSlotDecryptionSignature :exec
 INSERT INTO slot_decryption_signatures (eon, slot, keyper_index, tx_pointer, identities_hash, signature)
-VALUES ($1, $2, $3, $4, $5, $6);
+VALUES ($1, $2, $3, $4, $5, $6)
+ON CONFLICT DO NOTHING;
 
 -- name: GetSlotDecryptionSignatures :many
 SELECT * FROM slot_decryption_signatures