In AbstractIndexAccessMethod::insert/update and a few other places, we call shouldMarkIndexAsMultikey passing a vector of keys. Because the caller only has a set, it needs to copy all keys into a vector. However, shouldMarkIndexAsMultikey really only needs to know the number of keys, so we should stop passing the vector and doing the conversion.

Fixing this adds 3.3% to validation speed, and should speed up index operations in general.