Implement login resolver (#15)

* add sign_up_method to users table

* add session store

* implement login resolver

server/session/inMemoryStore.go

@@ -0,0 +1,41 @@
+package session
+
+import "sync"
+
+type InMemoryStore struct {
+	mu    sync.Mutex
+	store map[string]string
+}
+
+func (c *InMemoryStore) AddToken(userId, token string) {
+	c.mu.Lock()
+	// delete sessions > 500 // not recommended for production
+	if len(c.store) >= 500 {
+		c.store = make(map[string]string)
+	}
+	c.store[userId] = token
+	c.mu.Unlock()
+}
+
+func (c *InMemoryStore) DeleteToken(userId string) {
+	c.mu.Lock()
+	delete(c.store, userId)
+	c.mu.Unlock()
+}
+
+func (c *InMemoryStore) ClearStore() {
+	c.mu.Lock()
+	c.store = make(map[string]string)
+	c.mu.Unlock()
+}
+
+func (c *InMemoryStore) GetToken(userId string) string {
+	token := ""
+	c.mu.Lock()
+	if val, ok := c.store[userId]; ok {
+		token = val
+	}
+	c.mu.Unlock()
+
+	return token
+}