store_synchronize_sync(): Only sync subscribed folders.

store_synchronize_sync(): Only sync subscribed folders.

I recently added a Gmane (NNTP) account and have only a few newsgroup
subscriptions.  Yet when I quit Evolution, Camel tries to synchronize
all 13,872 newsgroups which literally takes days to complete.

This adds a CAMEL_STORE_FOLDER_INFO_SUBSCRIBED flag to the request for
a folder info tree while synchronizing + expunging.

Note, CamelIMAPXStore will ignore the CAMEL_STORE_FOLDER_INFO_SUBSCRIBED
flag if its "use-subscriptions" setting is FALSE, which is what we want.