archivers.aws: Make .05 a named constant
archivers.aws: Add helper method to return current time
util: Fix fractional portion of humanize_size() output
storage: Use humanize_size() for df output
archivers: Validate file size in ArchiveInfo.to_file()
archivers: Defer SHA-256 check from retrieve to unpack

and only if not checking a GPG signature.  Preparatory for random-access
retrieve.
archivers: Move ArchiveInfo.{from,to}_file() calls
archivers: Retrieve multiple archives in parallel
archivers: Drop ArchivePacker._pipeline()
archivers: During unpack, check for GPG signature by correct key
archivers.aws: Transfer data in 32 MB chunks
archivers.aws: Send retrieval summary and warning to stderr
rgw: Back up and restore bucket CORS configuration