resumablerequestreader: allow initial response

Make it possible to inspect an initial response and pass it to
ResumableRequestReader. This makes it possible to inspect an initial
response and passing it to ResumableRequestReader to avoid making an
extra request.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
get layer: remove HEAD req & pass down response

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
archive: add buffers to operations with tarballs

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
archive: add a benchmark for TarUntar

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Reorganize and clarify the DOCKER_BUILDTAGS docs

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
[DOCS] replace foo/bar with concrete names

namespaces are not well documented, and I had to jump around to other docs. replacing `foo/bar` hopefully makes what's going on here a bit more obvious.

Docker-DCO-1.1-Signed-off-by: Tim Ruffles <timruffles@gmail.com> (github: timruffles)
Merge pull request #6727 from unclejack/improve_bytes_range

Improve layer downloading
Merge pull request #7108 from unclejack/archive_speedup

speed up archive
Merge pull request #7181 from timruffles/patch-3

[DOCS] replace foo/bar with concrete names
Merge pull request #7148 from tianon/buildtags-docs

Reorganize and clarify the DOCKER_BUILDTAGS docs