Fix race condition causing unexpected idle disconnections

This issue may appear when 'send[data]' methods are triggered from different threads. As a result
threads race appears between cancelIdleTimeoutExecutor and scheduling new idle task.
Example of race:
1. Thread 1 cancels task
2. Thread 2 waits until thread 1 will finish
3. Thread 1 finishes
4. Thread 2 tries to cancel already cancelled task and leaves synchronized block
5. Thread 1 schedules new idle task
6. Thread 2 schedules another idle task, the first is overwritten and never cancelled.
As a result disconnection is triggered precisely after set idle timeout passes.

Signed-off-by: Mykyta Leonov <[email protected]>

core/src/main/java/org/glassfish/tyrus/core/TyrusSession.java

@@ -482,9 +482,8 @@ public void setHeartbeatInterval(long heartbeatInterval) {
  }
 
  void restartIdleTimeoutExecutor() {
- cancelIdleTimeoutExecutor();
-
  synchronized (idleTimeoutLock) {
+ cancelIdleTimeoutExecutor();
  idleTimeoutFuture =
  service.schedule(new IdleTimeoutCommand(), this.getMaxIdleTimeout(), TimeUnit.MILLISECONDS);
  }