Remove hs_index_t fetch, and use one of the stores instead
Split off from https://trac.torproject.org/projects/tor/ticket/23094#comment:9
Replying to [#23094 (moved):comment:3 asn]:
After our #23387 (moved) 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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