Make CamelNNTPStore a bit more thread-safe.

Make CamelNNTPStore a bit more thread-safe.

* Make CamelNNTPStore's public members private.

* Guard the private CamelNNTPStore members with a mutex.

* Add thread-safe accessors:

    camel_nntp_store_ref_cache()
    camel_nntp_store_ref_stream()
    camel_nntp_store_ref_summary()
    camel_nntp_store_get_current_group()
    camel_nntp_store_dup_current_group()
    camel_nntp_store_set_current_group()
    camel_nntp_store_add_capabilities()
    camel_nntp_store_has_capabilities()
    camel_nntp_store_remove_capabilities()

(cherry picked from commit fbf1ac4ac492951a3d6877e73961004741a0591c)