minor docfixes in ClusterSingletonManager.scala
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
Merge pull request #15145 from 2m/separate-sbt-files

=pro #15031 separate sbt build file for every module
Merge remote-tracking branch 'upstream/master'