Refactor internal/server/user package structure

Co-authored-by: Roberto Abdelkader Martínez Pérez <robertomartinezp@gmail.com>

internal/server/user/mux/mux.go

@@ -0,0 +1,30 @@
+package mux
+
+import (
+	"net/http"
+	"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()
+}
+
+func (sm *swappableMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	sm.get().ServeHTTP(w, r)
+}