Verify corefiles are the same via hash before using cached copy

[zachriggle]

Fixes #969

[zachriggle]

I've verified that this works correctly in a Docker instance.

Generating core-files of a running process, even without its state changing, will result in different SHA256 sums, so those will always be re-loaded.

Loading the corefile of a dead (dumped by the kernel) process correctly uses the cached copy.

[zachriggle]

@idolf This affects stable, but there's a new release on Sunday so we can just roll it all together then. I'm not sure if your workflow normally merges stable into beta before promoting beta to stable-staging or however you do it -- but this time, stable → beta merge does need to happen before release.

[TethysSvensson]

@zachriggle ACK