CDRIVER-466: Add mongoc_cursor_get_id()

These can be retrieved by calling currentOp, but its impossible to map
them to specific cursor. We can just as well expose the id rather then
having the user guessing

Closes #122

Author: bjori

src/mongoc/mongoc-cursor.c

@@ -1055,3 +1055,10 @@ mongoc_cursor_get_hint (const mongoc_cursor_t *cursor)
    return cursor->hint;
 }
 
+uint64_t
+mongoc_cursor_get_id (const mongoc_cursor_t  *cursor)
+{
+   BSON_ASSERT(cursor);
+
+   return cursor->rpc.reply.cursor_id;
+}

src/mongoc/mongoc-cursor.h

@@ -47,6 +47,7 @@ void             mongoc_cursor_set_batch_size (mongoc_cursor_t  *cursor,
                                                uint32_t          batch_size);
 uint32_t         mongoc_cursor_get_batch_size (const mongoc_cursor_t *cursor);
 uint32_t         mongoc_cursor_get_hint (const mongoc_cursor_t  *cursor);
+uint64_t         mongoc_cursor_get_id   (const mongoc_cursor_t  *cursor);
 
 
 BSON_END_DECLS