ThreadUtil performance improvement

The ThreadUtil changes introduced for the bug fix under JBTM-2673, cause lock contention on the static WeakHashMap under high load.

The proposed fix is to partially reintroduce the earlier behavior of caching the value in a ThreadLocal, since the dominant use case is pooled threads repeatedly looking up their own identity. The global lookup capability introduced for the fix remains necessary for less common use cases, but the locking can be taken off the critical path for most workloads.