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/repository.go

@@ -1,8 +1,6 @@
 package models
 
 import (
-	"context"
-
 	"github.com/stashapp/stash/pkg/file"
 	"github.com/stashapp/stash/pkg/txn"
 )
@@ -29,7 +27,3 @@ type Repository struct {
 	Tag         TagReaderWriter
 	SavedFilter SavedFilterReaderWriter
 }
-
-func (r *Repository) WithTxn(ctx context.Context, fn txn.TxnFunc) error {
-	return txn.WithTxn(ctx, r, fn)
-}