rename 'thingA' to something more permanent

[warner]

I created a data block temporarily named "thingA". Each uploaded file has an instance of this block. Each bucket maintains an identical copy of this block, and the hash of the block goes into the URI. This block is specified to be small (a few hundred bytes), independent of how large the corresponding file may be. This block contains any data necessary to reconstruct or validate the file that is not needed to locate the storage buckets. In particular, it contains merkle tree hash roots, full-file hashes, and certain encoding parameters (like segment size).

This block needs a more permanent name. "validation block" was proposed, as was "inode", "vnode", "metadata block", and others. I'd like for us to settle upon a name and change the code to match before making the next release. This affects RemoteInterfaces?, symbol names inside the code, and filenames on disk, so it is a significant compatibility-affecting change.

[warner]

we renamed it "URI extension block". source:docs/URI-extension.txt contains a description of what's inside it.