performance improvements

the heap refill code was recursively implemented with two methods.
I merged both methods.

replace recursion in heap refill method with iterative approach

use array for list of LongQueue
This way there is no precondition when accessing the elements

build.gradle

@@ -7,7 +7,7 @@ buildscript {
     jcenter()
   }
   dependencies {
-    // run with gradle --no-daemon clean jmh
+    // run with Java 11 and ./gradlew --no-daemon clean jmh
     classpath "me.champeau.gradle:jmh-gradle-plugin:0.5.2"
   }
 }
@@ -16,9 +16,13 @@ plugins {
   // usage: gradle dependencyUpdates -Drevision=release
   id "com.github.ben-manes.versions" version "0.34.0"
 }
+
 apply plugin: 'java'
 apply plugin: 'eclipse'
 
+// java compatibility version
+sourceCompatibility = 11
+
 /*
  * The shared configuration for all sub-projects:
  */
@@ -78,7 +82,7 @@ subprojects {
 	jmh {
 		//jvmArgsAppend = ""
 		resultFormat = "JSON"
-		//include = ['.*Union.*'] // include pattern (regular expression) for benchmarks to be executed
+		include = ['.*BenchmarkMultiwayMerge.*'] // include pattern (regular expression) for benchmarks to be executed
 		//exclude = ['some regular expression'] // exclude pattern (regular expression) for benchmarks to be executed
 	}
 }