Execute any function after each N iterations on `index:pairs()`

When iterating over a potentially large table, one needs to do something like:

local count = 0
for _, t in index:pairs(key) do
    process_tuple(t)
    count = count + 1
    if count == 100 do
        fiber.yield()
        count = 0
    end
end

We could provide a way to do it more conveniently. For example:

for _, t in index:pairs(key):after_each(100, fiber.yield) do
    process_tuple(t)
end

Such function could be used for other purposes:

-- This scenario requires to pass `count` parameter to the handler
for _, t in index:pairs(key):after_each(1000, function(cnt) log.info("Processed %d tuples", cnt) end) do
    process_tuple(t)
end

Or, instead of a general-purpose solution, we can just add a new option to pairs to handle only periodic yields:

for _, t in index:pairs(key, {yield_period = 100}) do
    process_tuple(t)
end