threadId keeps increasing reaching hardcoded limit of 64.

[enricogior]

When a new thread is created, a threadId from 1 to 63 is assigned to that thread.
When a thread exits, its threadId is not released, therefore the next thread created will have an increased threadId value. When the threadId reaches the value 64 the JXcore engine initialization for that thread will fail.

[enricogior]

The issue is now fixed. There is still the 64 threads limit, but now we recycle the thread id of the thread when it exits. More testing will be needed to make sure the fix hasn't unwanted side effects.