[Java] Simplify Java worker configuration (#2938)

## What do these changes do?
Previously, Java worker configuration is complicated, because it requires setting environment variables as well as command-line arguments.

This PR aims to simplify Java worker's configuration. 
1) Configuration management is now migrated to [lightbend config](https://github.com/lightbend/config), thus doesn't require setting environment variables.
2) Many unused config items are removed.
3) Provide a simple `example.conf` file, so users can get started quickly.
4) All possible options and their default values are declared and documented in `ray.default.conf` file.

This PR also simplifies and refines the following code:
1) The process of `Ray.init()`.
2) `RunManager`.
3) `WorkerContext`. 

### How to use this configuration?
1. Copy `example.conf` into your classpath and rename it to `ray.conf`.
2. Modify/add your configuration items. The all items are declared in `ray.default.conf`.
3. You can also set the items in java system prosperities.

Note: configuration is read in this priority:
System properties > `ray.conf` > `ray.default.conf`

## Related issue number
N/A

java/test/pom.xml

@@ -51,14 +51,6 @@
         <artifactId>maven-surefire-plugin</artifactId>
         <version>2.21.0</version>
         <configuration>
-          <environmentVariables>
-            <RAY_CONFIG>${basedir}/../ray.config.ini</RAY_CONFIG>
-          </environmentVariables>
-          <argLine>-ea
-            -Djava.library.path=${basedir}/../../build/src/plasma:${basedir}/../../build/src/local_scheduler
-            -noverify
-            -DlogOutput=console
-          </argLine>
           <testSourceDirectory>${basedir}/src/main/java/</testSourceDirectory>
           <testClassesDirectory>${project.build.directory}/classes/</testClassesDirectory>
         </configuration>