Support Acquire-By-Hash for index files

The added "aptly publish repo" option "-access-by-hash" publishes
the index files (Packages*, Sources*) also as hardlinked hashes.
Example:
 /dists/yakkety/main/binary-amd64/by-hash/SHA512/31833ec39acc...
The Release files indicate this with the option "Acquire-By-Hash: yes"

This is used by apt >= 1.2.0 and prevents the "Hash sum mismatch" race
condition between a server side "aptly publish repo" and "apt-get update"
on a client.
See: http://www.chiark.greenend.org.uk/~cjwatson/blog/no-more-hash-sum-mismatch-errors.html

This implementation uses symlinks in the by-hash/*/ directory for keeping
only two versions of the index files and deleting older files
automatically.

Note: this only works with aptly.FileSystemPublishedStorage

Closes: #536

Signed-off-by: André Roth <neolynx@gmail.com>

cmd/publish_repo.go

@@ -47,6 +47,7 @@ Example:
 	cmd.Flag.String("butautomaticupgrades", "", "set  value for ButAutomaticUpgrades field")
 	cmd.Flag.String("label", "", "label to publish")
 	cmd.Flag.Bool("force-overwrite", false, "overwrite files in package pool in case of mismatch")
+	cmd.Flag.Bool("access-by-hash", false, "provide index files by hash also")
 
 	return cmd
 }