Out of Memory a bridge too far

[Martin Pool]

I have some hope we can get streaming behaviour and lower memory usage
with the current formats.  Perhaps the fruit is not so high.

If we do need a special format for chunked or externally stored large
files we could use the recently landed feature flag thing to do it
with less trauma than previous transitions: you would only require a
new bzr to read repositories that actually have large files.

m