Lock stable_crate_ids once in create_crate_num

[zetanumbers]

This is a small refactor PR removing redundant locks of stable_crate_ids from create_crate_num.

[rustbot]

r? @jieyouxu

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[bjorn3]

This would take a write lock even when the crate already exists. The old code is racy though.

[zetanumbers]

This would take a write lock even when the crate already exists. The old code is racy though.

I thought using upgradable lock is too much to optimize the error path.

[jieyouxu]

@rustbot reroll

[bjorn3]

Doing a check with a read lock first and then taking a write lock and checking again is an option. If that is worth it, is another question.