We need a new name for this intentionally-vague block of data.

This block is a bencoded dictionary. All buckets hold an identical copy. The
hash of the serialized data is kept in the URI.

The download process must obtain a valid copy of this data before any
decoding can take place. The download process must also obtain other data
before incremental validation can be performed. Full-file validation (for
clients who do not wish to do incremental validation) can be performed solely
with the data from this block.

At the moment, this data block contains the following keys:

 size
 segment_size
 num_segments
 needed_shares
 total_shares

 codec_name
 codec_params
 tail_codec_params

 share_root_hash
 fileid
 plaintext_root_hash
 verifierid
 crypttext_root_hash


Some pieces are needed elsewhere (size should be visible without pulling the
block, the Tahoe3 algorithm needs total_shares to find the right peers, all
peer selection algorithms need needed_shares to ask a minimal set of peers).
Some pieces are arguably redundant but are convenient to have present
(test_encode.py makes use of num_segments).

fileid/verifierid need to be renamed 'plaintext_hash' and 'crypttext_hash'
respectively.

The rule for this data block is that it should be a constant size for all
files, regardless of file size. Therefore hash trees (which have a size that
depends linearly upon the number of segments) are stored elsewhere in the
bucket, with only the hash tree root stored in this data block.