minor docfixes in ClusterSingletonManager.scala
=act,tes Initial draft of internal MetricsKit

Note: This is NOT aimed to provide an micro-benchmarking solution.
The goal is to provide data for broad trend analysis. For techniques
that fight the inliner and other specialised techniques, refer to JMH.

+ custom console and graphite reporters
  - had to be custom because it's not possible to add custom metric
    types to the existing reporters
+ initial hdr.Histogram histogram() provider, see
  http://latencyutils.github.io/LatencyUtils/
+ Not using timers provided by Metrics, instead use the above histogram
+ Added average Actor size measurement
+ Measuring the "blocking time" when an actor is created, before we fire
  of the async part of this process; Measures in loop and will fluctuate
  a lot. Times are in `us` -- System.nanoTime should provide good enough
  resolution.
+ Measuring total actor creation time by using
  `KnownOpsInTimespanTimer`, which given a known number of ops, in a
  large amount of time, roughtly estimates time per one operation.
  // Yes, we are aware of the possibility of GC pauses and other horrors
+ All classes are `private[akka]`, we should not encourage people to use
  this yet
+ Counters use Java 8's `LongAdder`, which is metric's private;
  The new trend in Java land will be copy paste-ing this class ;)
+ Metrics are logged to Graphite, so we can long-term analyse these
+ Reporters are configurable using typesafe-config

! I'm not very happy about how I work around Metrics not being too open
  for adding additional custom metrics. Seems like a hack at places.
  I will consider removing the Metrics dependency all together.
  numbers

Example output:

```
-- KnownOpsInTimespanTimer-------------------------------------------
  actor-creation.total.creating-100000-actors.Props|new-EmptyArgsActor|…||-same
               ops = 100000
              time = 1.969 s
             ops/s = 50782.22
               avg = 19.69 μs

-- AveragingGauge---------------------------------------------------
  actor-creation.Props|new-EmptyArgsActor|…||-same.avg-mem-per-actor
                avg = 439.67
```
Merge pull request #15122 from ktoso/wip-metrics-kit

=act #15064 Initial draft of internal MetricsKit
=tes #15155 prettyduration should work with infinity
adjust default precision to be 4 (fits use case better)
=act #13964 make BalancingPool mailbox configurable
=doc #15160 Minor spelling, wording and grammar fixes.
Merge pull request #15160 from ectsue/patch-1

=doc #15160 Minor spelling, wording and grammar fixes.
Merge pull request #2127 from dormidon/3964-unable-to-configure-mailbox-for-routees-under-balancingpool-in-akka-2-3-1

Mailbox configuration for routees under BalancingPool - Issue 3964 fix
=act add one more test to routing.BalancingSpec

see #13964
Merge pull request #15134 from shikhar/patch-1

minor docfixes in ClusterSingletonManager.scala
Merge pull request #15156 from ktoso/tes-pretty-duration-inf

=tes #15155 prettyduration should work with infinity
=pro #15166 do not redefine executeTests
Merge pull request #15177 from 2m/fix-prolonged-mutlijvm-tests

Do not run multi-jvm tests twice
Merge pull request #15168 from akka/wip-readd-BalancingPool-test-∂π

=act add one more test to routing.BalancingSpec
=pro #15031 separate sbt build file for every module