Fix database locked errors (#3153)

* Make read-only operations use WithReadTxn
* Allow one database write thread
* Add unit test for concurrent transactions
* Perform some actions after commit to release txn
* Suppress some errors from cancelled context

pkg/models/mocks/transaction.go

@@ -9,7 +9,7 @@ import (
 
 type TxnManager struct{}
 
-func (*TxnManager) Begin(ctx context.Context) (context.Context, error) {
+func (*TxnManager) Begin(ctx context.Context, exclusive bool) (context.Context, error) {
 	return ctx, nil
 }
 
@@ -25,6 +25,9 @@ func (*TxnManager) Rollback(ctx context.Context) error {
 	return nil
 }
 
+func (*TxnManager) Complete(ctx context.Context) {
+}
+
 func (*TxnManager) AddPostCommitHook(ctx context.Context, hook txn.TxnFunc) {
 }