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/common_test.go

@@ -0,0 +1,17 @@
+package config_test
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func prepareExpectedConfig(t *testing.T, expectedCfg *model.Config) *model.Config {
+	// These fields require special initialization for our tests.
+	expectedCfg = expectedCfg.Clone()
+	expectedCfg.MessageExportSettings.GlobalRelaySettings = &model.GlobalRelayMessageExportSettings{}
+	expectedCfg.PluginSettings.Plugins = make(map[string]map[string]interface{})
+	expectedCfg.PluginSettings.PluginStates = make(map[string]*model.PluginState)
+
+	return expectedCfg
+}