lifetime of credentials

It seems gssproxy doesn't expose lifetime of credentials or doesn't do it properly.

In IPA env(WSGI, GSS_USE_PROXY=yes) I inquire the lifetime of creds as:
```
store = {'ccache': '/run/ipa/ccaches/xxx'}
creds = gssapi.Credentials(usage="initiate", name=None, store=store)
print(creds.lifetime)
```
which always show the initial lifetime of credentials (in my example it was always 20) even the credentials are expired.

While the decrypted ccache
```python
import gssapi

store = {'ccache': '/root/decryptedccache'}
creds = gssapi.Credentials(usage="initiate", name=None, store=store)
print(creds.lifetime)
```
shows the correct remaining lifetime of creds and raises with `ExpiredCredentialsError` on expiration.

Is such proxied lifetime's behaviour expected, bug or not implemented yet?