[MERGE] First cut at pluralised VersionedFiles. Some rather massive API incompatabilities, primarily because of the difficulty of coherence among competing stores.

[Andrew Bennetts]

Robert Collins wrote:
> Eep.
> 
> Key highlights:
>  - per file knit APIs: gone
>  - revision_store, control_store, weave_store, text_store: gone

Hooray for deleting code :)

>  - the original streaming api for repository and 'knits': gone (too big
>    a mismatch to sanely preserve I think. Happy to work on making it
>    exist if reviewers think its mandatory)

Well, at a minimum, we'll need new smart streaming pull verbs, or pull will
suddenly take far more network roundtrips.

Ideally we'd preserve the existing one too, to avoid massively penalising old
clients.  It might be easier to implement the verb than get_data_stream API, and
there'd be no need to implement insert_data_stream.

-Andrew.