[Spark 3922] Refactor spark-core to use Utils.UTF_8

A global UTF8 constant is very helpful to handle encoding problems when converting between String and bytes. There are several solutions here:

1. Add `val UTF_8 = Charset.forName("UTF-8")` to Utils.scala
2. java.nio.charset.StandardCharsets.UTF_8 (require JDK7)
3. io.netty.util.CharsetUtil.UTF_8
4. com.google.common.base.Charsets.UTF_8
5. org.apache.commons.lang.CharEncoding.UTF_8
6. org.apache.commons.lang3.CharEncoding.UTF_8

IMO, I prefer option 1) because people can find it easily.

This is a PR for option 1) and only fixes Spark Core.