HPCC-9595 Implement generalized paging/cache for DFU files

This fix replaces the existing paging/no cache function for
browsing logical files with new paging/cache function. The
MDFS_ITERATE_FILTEREDFILES flag and processing code are added to
support more filters on dali server side. In ESP side, a new method
getLogicalFilesSorted() and supporting code are added to set/send
the MDFS_ITERATE_FILTEREDFILES request with filters to dali server,
read the response into IPropertyTreeIterator for sorting, and sort/
cache the logical files. The old paging code is moved into a new
method which will be called for legacy dali.

3 minor changes: (1) Inside the response, replace file attribute
IsKeyFile to ContentType. (2) In dfu.xslt, rename the sort field
'Size' to 'FileSize'. (3) In dfu_search.xslt, merge the Sort Filter
FirstNType with FileSize.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>