Add ChannelLFStressTest

It tests lock-freedom on send and receive operations on rendezvous
and conflated channels. The test is comprehensive enough to fail on
buffered channels (which are currently not lock-free) and will help us
ensuring that lock-freedom property is not lost while channels are
being improved and refactored.
diff --git a/gradle.properties b/gradle.properties
index 6b3060e..316be1c 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1,3 +1,7 @@
+#
+# Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+#
+
 # Kotlin
 version=1.3.0-SNAPSHOT
 group=org.jetbrains.kotlinx
@@ -5,7 +9,7 @@
 
 # Dependencies
 junit_version=4.12
-atomicfu_version=0.12.10
+atomicfu_version=0.12.11
 html_version=0.6.8
 lincheck_version=2.0
 dokka_version=0.9.16-rdev-2-mpp-hacks