Revert "Allow workitems to startup faster and reduce polling thread interval in the threadpool."

This commit is causing unbound growth in the number of threads for a series of workloads.

It's leading a sgen test to crash with 800 tp threads around, which doesn't make sense on a
4 cores system with default TP configuration.

This reverts commit 3c347d5d3a9c58666ef33c96e28a16c751e59619.
Merge branch 'master' of github.com:mono/mono into symform-3-0