Include block size in metadata

[hannahhoward]

For metadata only requests, it may prove quite useful to add a BlockSize field to metadata, so we can factor raw bytes into decisions about how far to look ahead with fetching untrusted blocks.

[rvagg]

But we'd have to trust the peer to report the block size correctly, is that going to be a problem? I suppose we could check the size matches and abort requests if the peer reports anything wrong.