Refactor FsAdaptor.getDocumentContent: extracting methods

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

the retriever will be enhanced to support returning ACLs and
content given a docid that could be just a hostname, a sharepoint,
a DFS Namespace, a DFS link, a directory, or a regular file.
In all cases other than regular file, the content returned will
be generated HTML consisting of links to enumerated content,
in much the same way that directories are handled now.  For
shares, DFS Namespaces, and DFS links, ACL extraction and ACL
inheritance must be handled differently than with ordinary
files and directories.

In anticipation of these changes, this refactoring extracts
chunks of FsAdaptor.getDocumentContent() into separate methods.
The code for adding the filesystem ACLs, file and directory
content to the response were pulled out.

Future refactorings will target share ACLs and delay resolving
DFS links.

Code Review: https://codereview.appspot.com/179920043/