Force a refresh of the key cache after smart cards were learned

gpgsm learns the smart card certificates one by one. Since the key cache
watches the file system it does a refresh while gpgsm still learns more
certificates. This may cause the key cache to miss some of the learned
certificates. Explicitly forcing a refresh after gpgsm finished learning
the smart card certificates ensure that the cache knows all learned
certificates.

• GnuPG-bug-id: T6846