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 which
is similar to the function for browsing ECL WUs. The
MDFS_ITERATE_FILTEREDFILES process is added to support more
filters on dali server side. A new method getLogicalFilesSorted()
is 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.

This fix also adds ContentType for each file to indicate a key/csv/
xml/flat file.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>