[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/README.rst

@@ -1,18 +1,15 @@
-This directory contains the java worker, with the following components.
-
--  java/api: Ray API definition
--  java/common: utilities
--  java/runtime-common: common implementation of the runtime in worker
--  java/runtime-dev: a pure-java mock implementation of the runtime for
-   fast development
--  java/runtime-native: a native implementation of the runtime
--  java/test: various tests
--  src/local\_scheduler/lib/java: JNI client library for local scheduler
--  src/plasma/lib/java: JNI client library for plasma storage
-
 Quick start
 ===========
 
+Configuration
+-------------
+Ray will read your configurations in the following order:
+
+* Java system properties: e.g., ``-Dray.home=/path/to/ray``.
+* A ``ray.conf`` file in the classpath: `example <https://github.com/ray-project/ray/java/example.conf>`_.
+
+For all available config items and default values, see `this file <https://github.com/ray-project/ray/java/runtime/src/main/resources/ray.default.conf>`_.
+
 Starting Ray
 ------------