Always close key handle if key is persistent

[sbailey-arm]

Currently handles for persistent keys are kept open after generating and importing, which causes mbedtls to run out of key slots.

If a key is persistent, its handle should be closed after each generate and import operation.

Only set the handle field if the key is volatile.

[hug-dev]

Seems like a good thing to add to me!
Could you please also add an integration test where a big number of persistent keys are generated with both import and generate to see that it works?

[sbailey-arm]

Could you please also add an integration test where a big number of persistent keys are generated with both import and generate to see that it works?

Will do 👍

[hug-dev]

Done in linked PR