ClusterLoadBalancingRouter and refactoring of metrics, see #2547

* MetricsSelector, calculate capacity, weights and allocate weighted
  routee refs
* ClusterLoadBalancingRouterSpec
* Optional heap max
* Constants for the metric fields
* Refactoring of Metric and decay
* Rewrite of DataStreamSpec
* Correction of EWMA and removal of BigInt, BigDecimal
* Separation of MetricsCollector into trait and two classes,
  SigarMetricsCollector and JmxMetricsCollector
* This will reduce cost when sigar is not installed, such as
  avoiding throwing and catching exc for every call
* Improved error handling for loading sigar
* Made MetricsCollector implementation configurable
* Tested with sigar

akka-cluster/src/test/scala/akka/cluster/ClusterConfigSpec.scala

@@ -47,6 +47,7 @@ class ClusterConfigSpec extends AkkaSpec {
         callTimeout = 2 seconds,
         resetTimeout = 30 seconds))
       MetricsEnabled must be(true)
+      MetricsCollectorClass must be("akka.cluster.SigarMetricsCollector")
       MetricsInterval must be(3 seconds)
       MetricsGossipInterval must be(3 seconds)
       MetricsRateOfDecay must be(10)