FederatedOperationHandler Retains information from previous operation.

[GCHQDev404]

Describe the bug
The FederatedOperationHandler has retains a field called graphs and context which persists for the life of the Handler within the FederatedStore, retaining information from the previous execution.

Expected behaviour
Handler should not retain state from the previous execution.

[GCHQDev404]

Same issue happens with the context field within this class. But that was introduced earlier.

[t92549]

A potential solution is to just pass the graphs in to getAllGraphResults and mergeResults as parameters, and not keep them around at all between runs.