HPCC-9017 Add new open flags and copy logic to purge pgcache periodicall...

HPCC-9017

New *Flsh open flags and doCopyFile() code to flush page cache to prevent it from growing excessively.
If *Flsh flags are used then can still disable with conf file setting pgcache_flush=false

Signed-off-by: Mark Kelly <mark.kelly@lexisnexis.com>