HPCC-9577 - Handle sequential/parallel and refactor ifaction

With this commit ifaction/sequential/parallel can be generated
into the same graph as their actions.
This changes the way sinks are handled, to handle these 'sink'
activities and conditionally prepare/execute their inputs.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5132 from jakesmith/hpcc-9577c

    HPCC-9577 - Handle sequential/parallel and refactor ifaction

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>