This value is stored in the storage file header. We currently hard-code it to
0x1000. I don't expect to see files in the wild with other values, but
according to MS this is a valid configuration. For now, just fail if we see
another value.

I've also upgraded the message for unexpected values in storage file headers
to a fixme, since they are valid according to MS.

There's no need for this. Any useful information we could get out of it
is availble from the olemalloc channel, and it means that the ole channel
changes behavior in a way that's visible to programs.

This makes creating small block chains O(n) instead of O(n**2) because we
don't have to keep rechecking the first blocks in the file.