thpool: fix schedqueue licensing

clearly states where this code originates and the original licensing.
thpool: simplify the imports

since we want to support python >= 2.6 on modern platforms,
we can shed some legacy baggage.
thpool: use plain time.time()

since we are already bound by real wall clock due the usage
of Condition.wait(), there is no point of using a client-supplied
time function;

even worse, this can lead to potential inconsistencies, so drop it.
thpool: extracts the timeout sanity check

the timeout sanity check has to be performed anyway,
so the earlier, the better.
thpool: rewrite schedqueue

drop all the code not needed by thr watchedpool.
Ended up rewriting quite entirely.