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>