replace the quiet delay with a global state about nuxeo runtime thread usage

We've already added an API in the runtime for maintaining a state about the runtime uses of threads., ie:

org.nuxeo.runtime.trackers.concurrent

We could improve that API by defining a mandatory thread contract which prevent the runtime to be acceed if the thread has not acquired the right to.

This way, it will be possible to better coordinate threads at reload or shutdown without the use of a quiet delay.