This PR rewrites the `utils::result_parallel_for_each`'s implementation to resemble the original `seastar::parallel_for_each` more closely instead of using the less efficient `seastar::map_reduce`. It uses less tasks and allocations now, as demonstrated in the results from the `perf_result_query` benchmark, attached at the end of the cover letter.
The main drawback of the new implementation is that it needs to rethrow exceptions propagated as exceptional futures from the parallel sub-invocations. Contrary to the original `seastar::parallel_for_each` which uses a custom task to collect results, the new `utils::result_parallel_for_each` uses a coroutine and there doesn't currently seem to be a way to co_await for a future and inspect its state without either rethrowing or handling it in then_wrapped (which allocates a continuation). Fortunately, rethrowing is not needed for exceptions returned in failed result<>, which are already intended to be a more performant alternative to regular exceptions.
As a bonus, definitions from `utils/result.hh` are now split across three different headers in order to improve (re)compilation times.
Results from `perf_simple_query --smp 1 --operations-per-shard 1000000 --write` (before vs. after):
```
126872.54 tps ( 67.2 allocs/op, 14.2 tasks/op, 52404 insns/op)
126532.13 tps ( 67.2 allocs/op, 14.2 tasks/op, 52408 insns/op)
126864.99 tps ( 67.2 allocs/op, 14.2 tasks/op, 52428 insns/op)
127073.10 tps ( 67.2 allocs/op, 14.2 tasks/op, 52404 insns/op)
126895.85 tps ( 67.2 allocs/op, 14.2 tasks/op, 52411 insns/op)
127894.02 tps ( 66.2 allocs/op, 13.2 tasks/op, 52036 insns/op)
127671.51 tps ( 66.2 allocs/op, 13.2 tasks/op, 52042 insns/op)
127541.42 tps ( 66.2 allocs/op, 13.2 tasks/op, 52044 insns/op)
127409.10 tps ( 66.2 allocs/op, 13.2 tasks/op, 52052 insns/op)
127831.30 tps ( 66.2 allocs/op, 13.2 tasks/op, 52043 insns/op)
```
Test: unit(dev, debug)
Closes#10053
* github.com:scylladb/scylla:
utils/result: optimize result_parallel_for_each
utils/result: split into `combinators` and `loop` file
6b380121e0