[tor-bugs] #21039 [Core Tor/Tor]: Refactor and simplify guard code of circuit_send_next_onion_skin()

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#21039: Refactor and simplify guard code of circuit_send_next_onion_skin()
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     Reporter:  asn           |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.1.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-guard, refactor
Actual Points:                |  Parent ID:  #20822
       Points:  0.3           |   Reviewer:
      Sponsor:                |
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 As part of prop271 (#19877), we added about 70 fresh lines of code to
 `circuit_send_next_onion_skin()` which is an already convoluted function.

 Ideally we should abstract this new circuit-related guard code and put it
 in its own functions, to reduce complexity of
 `circuit_send_next_onion_skin()` and maybe even make it unittestable.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21039>
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