Use fair unlocking when locking the guard tracking mutex guard

[maackle]

PR summary

This may help with a case @lucksus saw where an instance couldn't acquire a lock for 20 seconds. The hypothesis is that running 80 instances in a single conductor caused the locksmith guard-tracking mutex to thrash and starve out one of the instances. Hopefully this helps.

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followups

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- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

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