MM-11262: database config store (#10281)

* vendor github.com/jmoiron/sqlx

* MM-11262: introduce a database store

* revert unnecessary fmt.Errorf

* simplify unit test helper methods

* remote TODO re: retry

* relocate initializeConfigurationsTable for clarity

* factor out a commonStore

* acquire database config lock on close for safety

* add missing header

* fix lock comment

config/store.go

@@ -4,6 +4,8 @@
 package config
 
 import (
+	"strings"
+
 	"github.com/mattermost/mattermost-server/model"
 )
 
@@ -39,3 +41,12 @@ type Store interface {
 	// Close cleans up resources associated with the store.
 	Close() error
 }
+
+// NewStore creates a database or file store given a data source name by which to connect.
+func NewStore(dsn string, watch bool) (Store, error) {
+	if strings.HasPrefix(dsn, "mysql://") || strings.HasPrefix(dsn, "postgres://") {
+		return NewDatabaseStore(dsn)
+	}
+
+	return NewFileStore(dsn, watch)
+}