bson_reader_reset()

According to the APM spec, server replies to OP_QUERY and OP_GETMORE must be converted into simulated replies to the "find" and "getMore" commands before reporting to the APM listener.

This requires reading the server reply twice: once to convert it to a find / getMore reply, once again while iterating the cursor normally with mongoc_cursor_next.

Add a public libbson function bson_reader_reset() to the cursor can re-read the server reply without a malloc.