… incomplete downloads

WriteLayer is an alternative to WriteBlob which can be used with
streaming layers which have not yet calculated a digest or size. A
temporary file is first written and then renamed when the reader has
been fully consumed. Also, existing layers which do not have the correct
size (and thus invalid hashes, but the hash is not checked) are
overwritten.

{Append,Replace,Write}{Image,Layer} functions utilize WriteLayer instead
of WriteBlob to correctly overwrite undersized layers which were
previously ignored without error. Undersized layers should no longer
occur, since WriteLayer writes to a temporary file first, but when
WriteBlob was used, these were common due to incomplete downloads.

…er blob content is truncated

…eLayers and temp file naming

…lds and fix data race