(autoload-find-destination): Understand a new format of autoload block where

(autoload-find-destination): Understand a new format of autoload block where
the file's time-stamp is replaced by its MD5 checksum.
(autoload-generate-file-autoloads): Use MD5 checksum instead of
time-stamp for secondary autoloads files.
(update-directory-autoloads): Remove duplicate entries.
Use time-less-p for time-stamps, as done in autoload-find-destination.