admission: SnapshotQueue can deadlock on a context cancellation race

[sumeerbhola]

This can happen if the context passed to the pacer for the incoming snapshot is cancelled, and there is a race with the granter calling into the SnapshotQueue.

Jira issue: CRDB-57829

[blathers-crl]

Hi @sumeerbhola, please add a branch-* label to identify the earliest affected branch for this C-bug

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