Refactor FsAdaptor share ACL handling to feed share ACLs at crawl time

In order to support the changes for the following bugs:

b/6513820: Enumerate share points or Namespaces given a hostname
b/12897821: Enumerate Namespace target links

Supplying only hostname or DFS Namespace as a startpoint
could eventually resolve to multiple shares being traversed.
In anticipation of these changes, this refactoring alters the
FsAdaptor code to feed the share ACLs at crawl time, rather
than in the lister.

It should be mentioned that this reverses a previous change
that moved feeding the share ACLs from the crawler to the
lister.  See https://codereview.appspot.com/14294044
Even though I performed that modification, the motivation
for doing so eludes me. IIRC, there were occasional
issues where the share ACL would not get fed, leaving a
broken inheritance chain for all documents.

Code Review: http://codereview.appspot.com/176110043