Add capability to set JHS job cache to a task-based limit

When setting the property mapreduce.jobhistory.loadedjobs.cache.size the jobs can be of varying size. This is generally not a problem when the jobs sizes are uniform or small, but when the job sizes can be very large (say greater than 250k tasks), then the JHS heap size can grow tremendously.

In cases, where multiple jobs are very large, then the JHS can lock up and spend all its time in GC. However, since the cache is holding on to all the jobs, not much heap space can be freed up.

By setting a property that sets a cap on the number of tasks allowed in the cache and since the total number of tasks loaded is directly proportional to the amount of heap used, this should help prevent the JHS from locking up.