"checkPresent baser" was wrong; the baser has a dummy checkPresent action
not the real one. So, to fix this, we need to call preparecheckpresent to
get a checkpresent action that can be used to check if chunks are present.
Note that, for remotes like S3, this means that the preparer is run,
which opens a S3 handle, that will be used for each checkpresent of a
chunk. That's a good thing; if we're resuming an upload that's already many
chunks in, it'll reuse that same http connection for each chunk it checks.
Still, it's not a perfectly ideal thing, since this is a different http
connection that the one that will be used to upload chunks. It would be
nice to improve the API so that both use the same http connection.
afe6a53bca