The previous implementation did some ratelimiting, namely how many parallel
ActivateConnection D-Bus calls are in fly. This way we are able to kick off
many parallel calls, but the activations themselves were not ratelimited.
Rework the code. Now there are two rate limits (that can be set via environment
variables):
NUM_PARALLEL_STARTING
NUM_PARALLEL_IN_PROGRESS
This allows more control about how much is happening in parallel. If we are
going to activate 1000 profiles, then it matters that we do things in parallel,
but not everything at the same time.
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