db IndexQuery preallocate 3 strides of items for the sets
db IndexQuery preallocate 3 strides of items for the sets

This should avoid many small allocations and save the size of those
buffers.
                                      old ns/op     new ns/op     delta
SingleIndexQueryFast-4                65516         52101         -20.4%
FiveIndexQueryFast-4                  591592        523803        -11.46%
InterleavedLeagueIndexQueryFast-4     877993        810060        -7.74%

                                      old allocs     new allocs     delta
SingleIndexQueryFast-4                315            293            -6.98%
FiveIndexQueryFast-4                  2310           2205           -4.55%
InterleavedLeagueIndexQueryFast-4     3696           3500           -5.30%

                                      old bytes     new bytes     delta
SingleIndexQueryFast-4                10715         7728          -27.88%
FiveIndexQueryFast-4                  87061         73579         -15.49%
InterleavedLeagueIndexQueryFast-4     144018        122041        -15.26%

Everlag/poeitemstore