[MERGE] Container format version 2

[Andrew Bennetts]

This primarily updates the container format to allow whitespace in record names.
The test suite uses file IDs with whitespace (e.g. TestFileIdInvolvedSuperset in
bzrlib/tests/repository_implementations/test_fileid_involved.py), and even if
the intent was to disallow it, bzrlib allows it and has for some time, so it's
probably not safe to assume there are no file IDs with whitespace in the wild.
So in this bundle the only disallowed byte in a record name is a '\n'.

I haven't made any effort to keep reading format 1 (or support generating it),
because it hasn't been used in any released code yet AFAIK.  If someone does
need this then I can spend some effort on this backwards compatibility, just
let me know.

-Andrew.