incremental verify for chunked remotes

Simply feed each chunk in turn to the incremental verifier.

When resuming an interrupted retrieve, it does not do incremental
verification. That would need to read the file, up to the resume point,
and feed it to the incremental verifier. That seems easy to get wrong.
Also it would mean extra work done before the transfer can start. Which
would complicate displaying progress, and would perhaps not appear to the
user as if it was resuming from where it left off. Instead, in that
situation, return UnVerified, and let the verification be done in a
separate pass.

Granted, Annex.CopyFile does manage all that, but it's not complicated
by dealing with chunks too.

Sponsored-by: Dartmouth College's DANDI project

doc/todo/OPT__58_____34__bundle__34___get_+_check___40__of_checksum__41___in_a_single_operation/comment_13_a19823409648828b8b9f7a0f2fa7d935._comment

@@ -0,0 +1,10 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 13"""
+ date="2021-08-11T18:16:41Z"
+ content="""
+Some special remotes now support incremental update. So far, limited to
+ones that use the byteRetriever interface.
+Others, that use fileRetriever, including external special remotes, still
+need work.
+"""]]