HPCC-8966 - Provide a context for GetGlobalUniqueIds

Add an IEngineContext for context code to conditionally use
where needed. The need 1st was for GetGlobalUniqueIds to
differentiate it's behaviour depending on engine context and
in particular whether it was being executed from a Thor slave
or not.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4096 from jakesmith/hpcc-8966

HPCC-8966 - Provide a context for GetGlobalUniqueIds

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>