fix threads loss

src/main/kotlin/io/emeraldpay/dshackle/upstream/AbstractHead.kt

@@ -53,13 +53,14 @@ abstract class AbstractHead(
                 val delay = System.currentTimeMillis() - lastHeadUpdated
                 if (delay > awaitHeadTimeoutMs) {
                     log.warn("No head updates for $delay ms @ ${this.javaClass} - restart")
+                    if (lock.tryLock()) {
                         try {
-                        lock.tryLock()
                             start()
                         } finally {
                             lock.unlock()
                         }
                     }
+                }
             }, 300, 30, TimeUnit.SECONDS
         )
     }