HPCC-7936 DFSAccess draft

Draft of the helper that should substitute CDistributedFileDirectory+CDistributedSuperFile in handling files and super-files in the DFS.

It will encapsulate all user, transaction, timeout and cache logic, as well as make it simple for external users to handle files without worrying about locks. It'll also introduce a number of validation checks to make sure we're not leaving the file system in an inconsistent state.

The goal is to make both DFUUtil and FileServices to use the DFSAccess, so that we don't need to fix the same problem twice.

Signed-off-by: Renato Golin <rengolin@hpccsystems.com>