[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/pom.xml

@@ -95,6 +95,11 @@
         <artifactId>slf4j-log4j12</artifactId>
         <version>${slf4j.version}</version>
       </dependency>
+      <dependency>
+        <groupId>com.typesafe</groupId>
+        <artifactId>config</artifactId>
+        <version>1.3.2</version>
+      </dependency>
       <dependency>
         <groupId>net.java.dev.jna</groupId>
         <artifactId>jna</artifactId>