Reinstate zeroisation of private keys

[jarhodes314]

In #3371, I had to remove the zeroisation of private keys due to a change in rustls_pki_types preventing modification of the data. We should attempt to reinstate this by creating a PR for a zeroize feature in rustls_pki_types.

[jarhodes314]

I've created rustls/pki-types#71 to deal with this, and this is already being merged. We'll need to wait until there's a release to upgrade and then make some changes on our side to call zeroize, but this should be possible soon.