Delay creating knit contents

[John Arbash Meinel]

The attached patch changes when Knit files create their first file
contents. So that instead of being created when you instantiate them,
they are only created when the first data is put into the files.

This has a pretty large impact on pushing over sftp. Specifically
without this patch:

SFTPSlowSocketBenchmark.test_initial_push   OK 167569ms/178929ms

but with the patch:

SFTPSlowSocketBenchmark.test_initial_push   OK 105947ms/117429ms

So that is taking it down from 160s down to 100s. I haven't spent a lot
of time profiling the new code, as I thought the fix would have had an
even greater effect. But right now, I'd like to just get what I have
merged, and then I can look into making it even better.

This also affects 'bzr commit' time.:
CommitBenchmark.test_commit_kernel_like_tree   OK 50614ms/54579ms
vs
CommitBenchmark.test_commit_kernel_like_tree   OK 39780ms/46376ms

So that is a 25% improvement for committing 10K new files.

John
=:->

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