HPCC-9595 Implement generalized paging/cache for DFU files

The existing Browse Logical Files code does not have cache function.
It uses its local paging code. This fix implements a paging/cache
mechanism similar to Browse ECL WU page.
This fix also made two small changes: 1. Add ContentType for each
file to indicate a key/csv/xml/flat file; 2. Rename flag IsKeyFile
to IsCompressed (keep old one for legacy);

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