Allow negative timeouts in all wait functions

[Gerold103]

For example, netbox future:wait_result() fails on a negative timeout with an exception instead of just returning nil,err. That is quite unhandy. Consider this example (from vshard):

    for uuid, future in pairs(futures) do
        res, err = future:wait_result(timeout)
        -- Handle netbox error first.
        if res == nil then
            err_uuid = uuid
            goto fail
        end
        -- Ref returns nil,err or bucket count.
        res, err = res[1], res[2]
        if res == nil then
            err_uuid = uuid
            goto fail
        end
        bucket_count = bucket_count + res
        timeout = deadline - fiber_clock()
    end

Here I don't want to care whether timeout is 0 or -0.000001 (in this case I just missed it). I don't want to check it explicitly. Code is 5 lines shorter in this case when I can trust :wait_result() to gracefully return an error on a negative timeout. Otherwise I would need to add:

if timeout < 0 then
    err = box.error.new(box.error.TIMEOUT)
    err_uuid = uuid
    goto fail
end

(I will need to add it now anyway, because I have to support old versions too.)

The problem grows when I have many :wait_result() usages and have to invent one another vshard.util function to make it easier to use.

AFAIR the same problem is at least with our condition variables.

It might even be called inconsistent, because we legally allow to have negative timeouts - clock_* functions in clock.lua return int64_t specifically not to care about accidentally going below zero.