Run async tasks that access AddressBook threaded mutexes on dedicated threads

[teor2345]

Motivation

zebra-network keeps on having hard-to-diagnose hangs. These hangs could be happening because the AddressBook uses a threaded mutex, which blocks the executor, and all async tasks scheduled on the same thread.

Designs

• launch tasks that access the AddressBook mutex using spawn_blocking, so they get their own dedicated thread
  • if needed, make the method async, or make it return a Future
  • if that's not possible, use block_in_place (this blocks tasks on the same thread, but not the whole executor)

Related Work

• temporary fix for Turn the CandidateSet and AddressBook into tower Services #1976

[mpguerra]

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[teor2345]

FYI this is implemented in PR #3244.