HPCC-8465 Implement THISNODE(dictionary) and serialize to slaves

Stage one of the work to improving slave serialization.  I think this
is correct as far as it goes.  However the dictionaries are serialized
to the slaves as datasets which is inefficient.  That needs revisiting.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>