Made use of mutex in a goroutine that cleans expired captchas.

pkg/captcha/db.go

@@ -55,6 +55,8 @@ func (cdb *CaptchaDB) SetExpiry(expire time.Duration) {
 
 			for id, captcha := range cdb.DB {
 				if time.Now().Sub(captcha.Expiry()) <= 0 {
+					cdb.Lock()
+					defer cdb.Unlock()
 					delete(cdb.DB, id)
 				}
 			}