WorkManager shutdown should give a chance to work to suspend

On shutdown, the work manager should wait a fixed amount of time between setting the suspending flag and shutting down the pool.

We could do smarter things like adding a state between suspending and suspended and wait for work that suspending in progress but at the end, there will be a timeout, so wait for a fixed amount of time like 20s might be good enough.

This delay should be configurable and set by default to 0s, only instance that handle long running work should increase this delay.

This should also be done for the StreamWorkManager and backported in 9.10