Previous `ForEach()` required transforming the input into a slice of
interfaces (causing an allocation on heap for each job, although that's
minor here), and type asserting each job in the function later (generics
will save us, but they're still not here). Plus since job index is not
known, most use cases require a mutex to concatenate the results.

This replaces it with `ForEachJob` instead, which provides the index of
the job to `jobFunc` instead, letting it decide how to handle it. In
most cases that usage will be accessing the original jobs slice, with
proper type checking. Given the index of the job is known, no mutex is
needed to store the result in a slice afterwards (see test change), plus
since indexes are just ints, we don't even need a channel.

This fits all the `concurrency.ForEach` uses I could find, and since
there's no released version of dskit yet, I just removed that function.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Co-authored-by: Peter Štibraný <peter.stibrany@grafana.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>