Metric for active ipc Readers and make default fraction of cpu count

Our ipc Readers are hard coded at 10 regardless since . Running w/ less Readers, we go faster..(e.g. 12 Readers has us doing 135k with workloadc and 6 readers has us doing 145k).. .but hard to tell what count of Readers are needed since no metric.

This issue changes Readers to be 1/4 the installed CPUs or 8, whichever is the minimum, and then adds a new hbase.regionserver.ipc.runningReaders metric so you have a chance seeing whats needed.