Create AbusiveUserMapper to replicate an abusive user's workload

[csgregorian]

Added a new WorkloadMapper, AbusiveUserMapper, that creates two types of tasks: small operations (single file listing or single folder creation) and large listing operations (configurable # of files). This is useful to see the interaction of users that have dramatically different performance impacts and resource usage. The proportion of small to large/abusive users and the number of mappers for each is configurable.