Implement short retries in single threaded context

[sosthene-nitrokey]

Also, fix round retry mechanism not being used for login This two are merged because the changing of the next_instance signature showed that login() did not use the retry mechanism.

This does not count in the timeout calculation for each requests, so requests are limited to perform up to 3 health check at most (each with 1s timeout, so 3 seconds total).

This also makes all tests run both in single-threaded an multi-threaded modes.

This also removes some memory leaks in tests and in the Finalize call (the background thread did not close the instances it had.

[sosthene-nitrokey]

Once this is merged, we can fix #209:

• Failing Instances are marked as bad and retried in a background thread, so they won't be tried unless all instances are unreachable
• If no background thread can be spawned (CKF_LIBRARY_CANT_CREATE_OS_THREADS), failed instances will be tried during normal operations, slowing down the requests. To minimise this, such "inline" health checks are limited to 1 seconds timeouts, and only 3 health checks can be attempted per request (can only be reached if a large numbe of instances are failed).
• The total number of requests is: num_retries + 1
• The total timeout for 1 request attempt is: (num_retries + 1)*timeout_seconds + 3
• The total timeout for 1 PKCS11 function call will vary because some functions will lead to multiple API calls in the nethsm.

[sosthene-nitrokey]

@robin-nitrokey @daringer can you please review ?