[tor-bugs] #25964 [Core Tor/Tor]: Remove hs_index_t fetch, and use one of the stores instead

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#25964: Remove hs_index_t fetch, and use one of the stores instead
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     Reporter:  teor          |      Owner:  (none)
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  technical-debt, refactor
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Split off from
 https://trac.torproject.org/projects/tor/ticket/23094#comment:9

 Replying to [#23094:comment:3 asn]:
 > After our #23387 refactoring of hsdir index logic Nick suggested that we
 don't need to keep all 3 around in memory, since two of them are always
 identical:
 > https://oniongit.eu/asn/tor/merge_requests/6#note_1201

 We need to:
 * analyse the state machine of fetch, store_first, and store_second to
 make sure that fetch is always equal to store_first or store_second
 * work out the condition that we can use to select betweeen store_first or
 store_second when we want fetch
 * write a function that produces fetch from a hsdir_index_t, and use it
 instead of fetch

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25964>
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