info: Generate NAS by links rather than access

Info generates Normalized Access Storages - a non-overlapping grouping of pools
that share some properties.

In the past, how pools were grouped was based on which storage and dcache units
could do what kind of operation on the pool. Pools with the same access such
access properties were put into the same NAS. This did however result in NAS
that were rather difficult to understand: They had weird names that encoded the
access rules, and the list of links for the NAS did not apply to all pools in
the NAS. Also, the definition ignored link groups, which meant that pools
belonging to different link groups could be put into the same NAS if they
otherwise shared the same access properties.

This patch updates the NAS generator. It now groups those pools that are
accessible through the same set of links. NAS names are now an ordered list of
link names, which makes the relation ship to the pool manager configuration
much more obvious. The links of a NAS provide access to all the pools in the
NAS.

To the reviewer (Paul in particular): Please double check the updated unit
tests.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7305/