Feature sync: support partial map-reduce

Original: https://github.com/KaymeKaydex/go-vshard-router/issues/118

tnt vshard router supports partial map-reduce, we don't support it yet:

https://github.com/tarantool/vshard/commit/63fb7f79484a4f3fa4185cf3db3d1375919ad2c8

https://github.com/tarantool/vshard/commit/e6e7d3883f81945ee915c0fafa41a8e362cab993