apmbench: Support sending the benchmark data in a random order

[marclop]

Description

apmbench currently sends the data in the same order as it's loaded, while this works fine for now, it would be interesing to add support to the benchtest/eventhandler package to allow shuffling the data when the slice is iterated over.

Possible solution

We could add a -shuffle flag to apmbench which may use math/rand.Perm to randomize the access to the slice of readers that is used to send the data to the APM Server.