Globus cert verification does not clear proxy_depth or max_proxy_depth (unique_ptr)

[jthiltges]

Issue is already being discussed in PR #9. Creating a new request as this is a different implementation using unique_ptr.

Details:

• proxy_depth is incremented and never cleared during validation
• max_proxy_depth is modified if the proxy has a path-length limit set

After a context validates a certificate with a path-length limit, subsequent validations may fail with "Proxy path length exceeded" errors.

Allocate a new callback struct with unique_ptr for each validation.