Adapt plugincache MergeStrategy to sbt-assembly:2.0.0

Update MergeStrategy code to use temp files to extract the InputStreams
Update the sbt assembly plugin to use 2.0.0-RC1
Update the log4j version to 2.17.2
Update version to 2.1.0

src/main/scala/sbtassembly/Log4j2MergeStrategy.scala

@@ -27,7 +27,6 @@ object Log4j2MergeStrategy {
         () => new BufferedInputStream(new FileInputStream(mergedDatFile))
       }
     }
-    datFiles.foreach(IO.delete)
     Right(JarEntry(dependencies.head.target, stream) +: Vector.empty)
   }
 }