An instance of pool is created per thread and it stores a reference to the
current pool values.
When the pool is initialized, a temporary reference is set on the instance so
only the values for this thread is modified. Once the initialization is
complete, the new pool value replace the global values.
Each instances is refreshed with the global values after the pool has been
stopped.
Closes#11401
Cédric Krier
ec5a00c82e