Clean up Live Development agent loading

Currently, Live Development agents resolve their `load()` promises before remote services they rely on have been completely enabled. For example, `NetworkAgent.load()` calls `Inspector.Network.enable()` but does not wait for this asynchronous call to complete before returning. This pull request delays completion of these load calls until the necessary asynchronous calls have successfully completed.

I'm not aware of any problems caused by the current behavior, but fewer race conditions seems better than more race conditions. Startup races like this also tend to manifest themselves during automated testing.