Rename the members of view_update_builder to reflect their roles
more precisely:
_updates -> _update_reader
_existings -> _existing_reader
_update -> _update_fragment
_existing -> _existing_fragment
This makes the code easier to follow by distinguishing the readers
(which produce a stream of fragments) from the cached fragments
(the most recently read mutation_fragment_v2 from each reader).
490b3f5c6f