Reindexing nodes should remove existing values from indexes

ATM when issuing calls to workspace.reindex, none of the existing indexed values are removed. This means that duplicate indexes would grow out-of-control with each reindexing operation.
We need to update the SPI to make sure that when reindexing is performed (either for an entire workspace or a subgraph), the index providers are asked to first clear the values for the corresponding nodes.