Proposed new Java feature

First a few observations:

1. ThreadLocals may be, in general, an abomination, but there are situation
where they're the least of evils. And if you're building a framework in
which third-party code runs (e.g. a webserver), there's no way to disallow
their use.

2. ThreadLocals interact badly with ThreadPools, because the ThreadLocals
keep their value when the tyhread is put back into the pool. This can lead
to leaks and even potential security issues.

3. The current implementation of ThreadLocal is this: each thread contains a
map from the ThreadLocal instance to its value for that thread. (This is
slightly less intiutive than giving a ThreadLocal instance a map from Thread
to value, but has the advantage that the map, which is only used within one
thread, needn't be synchronized.)

Proposed feature: a static method on Thread that clears all ThreadLocals for
the current thread. By 3 it's trivial to implement. By 1 and 2 it's
needed. And ThreadPool implementations can simply call it directly before
putting the Thread back into the pool.