Locks are not exception safe

[RasmusSkytte]

Describe the bug
Locks are not exception-safe. update_snapshot() and delta_load() acquire locks but only release them at normal-function exits. Any error or interrupt between lock and unlock can leave stale locks.

To Reproduce
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Expected behaviour
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Screenshots
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Provide output from sessionInfo()
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• I have checked that the bug is not already described in open issues

Additional context
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