Fixed mutexes. Now program doesn't fall under high load.

Seems like an old expiry comparing didn't work as left thousands of expired captchas in a map, so it was replaced by a forgotten by me time.Since() function.

pkg/captcha/db.go

@@ -53,13 +53,13 @@ func (cdb *CaptchaDB) SetExpiry(expire time.Duration) {
 			sleepFor := expiredScanInterval - (time.Duration(time.Now().Second()) % expiredScanInterval)
 			time.Sleep(sleepFor)
 
+			cdb.Lock()
 			for id, captcha := range cdb.DB {
-				if time.Now().Sub(captcha.Expiry()) <= 0 {
-					cdb.Lock()
-					defer cdb.Unlock()
+				if time.Since(captcha.Expiry()) >= cdb.ExpireIn {
 					delete(cdb.DB, id)
 				}
 			}
+			cdb.Unlock()
 		}
 	}()
 }
@@ -71,14 +71,16 @@ func (cdb *CaptchaDB) New(data string, captcha ICaptcha) (ICaptcha, ID) {
 	id := NewID(data, captcha.GetAnswer())
 
 	cdb.Lock()
-	defer cdb.Unlock()
 	cdb.DB[id] = captcha
+	cdb.Unlock()
 
 	return captcha, id
 }
 
 // Image returns image for a captcha.
 func (cdb *CaptchaDB) Image(id ID, style string) (*image.Image, error) {
+	cdb.Lock()
+	defer cdb.Unlock()
 	if c, ok := cdb.DB[id]; ok {
 		return c.GetImage(style), nil
 	}