perf: each subscription is awaited making each of them sequential and possibly affecting performance

[thlorenz]

With the latest subscription correctness fix we introduced a possible bottleneck. Due to this we now await each subscription to be completed before handling subsequent subscriptions.

This could lead to subscription requests queuing up and a latency spike.

We can fix this by processing each subscription request in parallel via unordered futures while still guaranteeing that the code initiating a specific subscription doesn't continue until the subscription is realized.