libstore: Make writable dummy store thread-safe

[xokdvium]

Motivation

Tested by building with b_sanitize=thread and running:

nix flake prefetch-inputs --store "dummy://?read-only=false"

It might make sense to move this utility class out of dummy-store.cc,
but it seems fine for now.

Context

Fixes #14036
Follow-up to #14023

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[edolstra]

I think defining hash_value is unnecessary if you add

using is_avalanching = std::true_type;

to struct hash<nix::StorePath>.

[xokdvium]

That is unrelated:

When using a hash function directly suitable for open addressing, post-mixing can be opted out by via a dedicated hash_is_avalanchingtrait. boost::hash specializations for string types are marked as avalanching.

https://www.boost.org/doc/libs/1_85_0/libs/unordered/doc/html/unordered.html

Also this isn't really the case here.

A hash function is said to have the avalanching property if small changes in the input translate to large changes in the returned hash code

Because we just take the first 8 bytes of the hash part as the hash.

[xokdvium]

Also it doesn't help with the build anyway:

boost/container_hash/hash.hpp:537:20: error: no matching function for call to 'hash_value'
  537 |             return hash_value( val );

[xokdvium]

https://www.boost.org/doc/libs/latest/libs/container_hash/doc/html/hash.html#notes_the_hash_value_customization_point