db encodeModIndexKey reduce to single allocation
db encodeModIndexKey reduce to single allocation

Previously, we had multiple buffers involved in mod index key
generation, now we have only a single.

                            old ns/op     new ns/op     delta
CompactAddStashesFast-4     164564120     157177830     -4.49%
CompactFast-4               74662715      74324415      -0.45%
AddStashesFast-4            87291400      86455555      -0.96%

                            old allocs     new allocs     delta
CompactAddStashesFast-4     832610         806239         -3.17%
CompactFast-4               243723         243723         +0.00%
AddStashesFast-4            587242         562387         -4.23%

                            old bytes     new bytes     delta
CompactAddStashesFast-4     44618360      44372068      -0.55%
CompactFast-4               22287875      22287472      -0.00%
AddStashesFast-4            21493422      21958668      +2.16%

Everlag/poeitemstore