agent: fix possible deadlocks of queue overloading when processing messages (#1713)

* agent: fix possibly deadlocks of queue overloading when processing messages

* simplify

* refactor

* refactor

* refactor 2

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Co-authored-by: Evgeny @ SimpleX Chat <259188159+evgeny-simplex@users.noreply.github.com>

Author: epoberezkin