Move swappableMux and its methods to its own module

internal/server/user/mux.go

@@ -0,0 +1,25 @@
+package user
+
+import (
+	"sync"
+
+	"github.com/gorilla/mux"
+)
+
+type swappableMux struct {
+	m    sync.RWMutex
+	root mux.Router
+}
+
+func (sm *swappableMux) get() mux.Router {
+	sm.m.RLock()
+	defer sm.m.RUnlock()
+
+	return sm.root
+}
+
+func (sm *swappableMux) set(mux mux.Router) {
+	sm.m.Lock()
+	sm.root = mux
+	sm.m.Unlock()
+}