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>

api/publish.go

@@ -238,6 +238,7 @@ func apiPublishUpdateSwitch(c *gin.Context) {
 			Component string `binding:"required"`
 			Name      string `binding:"required"`
 		}
+		AccessByHash *bool
 	}
 
 	if c.Bind(&b) != nil {
@@ -317,6 +318,10 @@ func apiPublishUpdateSwitch(c *gin.Context) {
 		published.SkipContents = *b.SkipContents
 	}
 
+	if b.AccessByHash != nil {
+		published.AccessByHash = *b.AccessByHash
+	}
+
 	err = published.Publish(context.PackagePool(), context, context.CollectionFactory(), signer, nil, b.ForceOverwrite)
 	if err != nil {
 		c.AbortWithError(500, fmt.Errorf("unable to update: %s", err))