To be future proof, I think we should allow the SPI to return multiple write-to indexes.

CapeDwarf is going to need it, and also I was thinking of cases in which people want to potentially feed some Documents into multiple indexes.
Could be interesting for:
 - have a leaner index which contains only some core data, for example not supporting projections but favoring certain queries
 - write both into a remote indexing service and a local index copy
...