Fix rollback errors in logs (stellar#397)

During ingestion we can see the following logs:

WARN[2023-02-06T21:16:22.733+01:00] could not rollback ingest write transactions  error="sql: transaction has already been committed or rolled back" pid=78232

This commit will ignore sql.ErrTxDone errors when rolling back a transaction.

cmd/soroban-rpc/internal/db/db.go

@@ -168,7 +168,14 @@ func (w writeTx) Commit(ledgerSeq uint32) error {
 }
 
 func (w writeTx) Rollback() error {
-	return w.tx.Rollback()
+	// sql.ErrTxDone is returned when rolling back a transaction which has
+	// already been committed or rolled back. We can ignore those errors
+	// because we allow rolling back after commits in defer statements.
+	if err := w.tx.Rollback(); err == nil || err == sql.ErrTxDone {
+		return nil
+	} else {
+		return err
+	}
 }
 
 func runMigrations(db *sql.DB, dialect string) error {

cmd/soroban-rpc/internal/db/ledger_test.go

@@ -67,6 +67,8 @@ func TestLedgers(t *testing.T) {
 		assert.NoError(t, err)
 		assert.NoError(t, tx.LedgerWriter().InsertLedger(createLedger(ledgerSequence)))
 		assert.NoError(t, tx.Commit(ledgerSequence))
+		// rolling back after a commit is a no-op
+		assert.NoError(t, tx.Rollback())
 	}
 
 	assertLedgerRange(t, reader, 1, 10)