Include manifest length to the digest file.

To speed up parsing we need to know the exact length of manifest pushed
into packagesite bulk. Therefore, this patch modifies the digest file
format from:

<origin>:<digest>:<manifest_offset>:<file_offset>

to:

<origin>:<digest>:<manifest_offset>:<file_offset>:<manifest_length>

This does not break compatibility with the older versions of pkg and
improve the packagesite parsing speed.