DocumentExchangerInterface should also allow fetching multiple documents at once

[damienalexandre]

From a performance POV, the current DocumentExchangerInterface is not ideal for the MultipleIndexationRequest use-case.

It can only fetch one document at a time, so it perform 1 SQL request (if SQL is used) for each IndexationRequest instead of possibly one for the whole MultipleIndexationRequest.

Would be nice to add DocumentExchangerInterface::fetchDocuments and use it when appropriate.