HPCC-8624 - Ensure activities that read a logical keep lock

Not all of the file reader activities were holding onto to the
IDistributed file (and hence the Dali lock). Consequently, it
would be possible for someone to delete a logical file that a
Thor query was actively using.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4873 from jakesmith/hpcc-8624

HPCC-8624 - Ensure activities that read a logical keep lock

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>