Consolidate the saving of object dependencies and saving the actual
objects' data into a single function called `add_objects`. The previous
methods `add_dependencies`, `add_custom_invalidation_data` and `put`
have been removed.
Split the `invalidate` method in two phases: in `invalidateCheck` it
just checks if the model diff warrants the execution of the actual
`invalidate` method, which requires fetching the custom invalidation
data from storage. The actual `invalidation` method remains the same.
This should make invalidation faster for storage providers that store
the invalidation remotely.
In preparation for debugging individual pipes add an additional method
to the `Pipe` interface, `needed_files`, which allows the pipe to
statically report which files it will request ahead of time.
Giacomo Vercesi