Block hot reloads on pending deferred loading requests

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Deferred loads and hot reloads are two mechanisms that can add new libraries into a running application. While a deferred load cannot start during a hot reload, a hot reload can start during a deferred load. This can be problematic if both mechanisms end up loading two different versions of the same library.

Rather than dealing with resolving the multiple versions and risking loading the wrong one, it will be simpler to just avoid the problem in the first place. We should block any hot reloads from starting until any active deferred loads have completed.