JBTM-2808 adds synchronization on ThreadUtil.getThreadId() but that solution does not scale well enough under high contention.

There seems to be ways to remove this synchronization:

1. ConcurrentWeakHashMap - replace the current WeakHashMap for a similar data structure that can handle concurrent access. This keeps the same behavior.
2. Use Thread.getId() - remove all mappings and generate the threadId from the Thread. Although the javadoc mentions that IDs can be reused, the implementation is a counter, like what ThreadUtil has now. This is the simplest and most elegant fix.
3. Remove threadId - remove the generation of threadId and use the Thread throughout the code, generating string IDs when necessary. Probably yields the best performance, but requires the most changes.