add, optional, in memory password caching

Verifying argon2 passwords has a high memory and computational cost,
by enabling, in memory, password caching you reduce this cost

dataprovider/cachedpassword.go

@@ -0,0 +1,60 @@
+package dataprovider
+
+import "sync"
+
+var cachedPasswords passwordsCache
+
+func init() {
+	cachedPasswords = passwordsCache{
+		cache: make(map[string]string),
+	}
+}
+
+type passwordsCache struct {
+	sync.RWMutex
+	cache map[string]string
+}
+
+func (c *passwordsCache) Add(username, password string) {
+	if !config.PasswordCaching || username == "" || password == "" {
+		return
+	}
+
+	c.Lock()
+	defer c.Unlock()
+
+	c.cache[username] = password
+}
+
+func (c *passwordsCache) Remove(username string) {
+	if !config.PasswordCaching {
+		return
+	}
+
+	c.Lock()
+	defer c.Unlock()
+
+	delete(c.cache, username)
+}
+
+// returns if the user is found and if the password match
+func (c *passwordsCache) Check(username, password string) (bool, bool) {
+	if username == "" || password == "" {
+		return false, false
+	}
+
+	c.RLock()
+	defer c.RUnlock()
+
+	pwd, ok := c.cache[username]
+	if !ok {
+		return false, false
+	}
+
+	return true, pwd == password
+}
+
+// CheckCachedPassword is an utility method used only in test cases
+func CheckCachedPassword(username, password string) (bool, bool) {
+	return cachedPasswords.Check(username, password)
+}