filetransfer: make two-arg download abstract

this lets each implementation of FileTransfer (of which currently only
the one exists at all) implement appropriate handling for its internal
behaviours that are not otherwise exposed. in curl this lets us switch
the buffer-full handling method from "block the entire curl thread" to
"pause just the one transfer", move the non-libcurl body decompression
out of the actual curl wrapper (which will let us eventually morph the
curl wrapper intto an actual source of Sources), and some other things

Change-Id: Id6d3593cde6b4915aab3e90a43b175c103cc3f18

0 files changed, 0 insertions, 0 deletions