Support by-hash mirrors

[nthykier]

So we can avoid needing to wait for the dinstall/dak lock.

[nthykier]

InRelease parsing:

import os
import apt_pkg

try:
  fd = apt_pkg.open_maybe_clear_signed_file('InRelease')
  tagFile = apt_pkg.TagFile(fd)
  ...  
finally:
  os.close(fd)