This dedicated pool class shared by all the actors instantiating Object Actors
(console, thread and tracer actors) is going to later help reuse the same
WeakMap logic to instantiate only one actor per unique JS value.
I also merged console and target actor to use the same pool
as they basically have the same lifecycle. This simplify lookups
by Actor IDs done for selected object actor or console table objects.
Differential Revision: https://phabricator.services.mozilla.com/D225235