Implement deleteTask API

[alirezazamani]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Fixes Delete a tasks from the running (IN-PROGRESS) jobs or the jobs that have not started yet #1467

Description

• Here are some details about my PR, including screenshots of any UI changes:
  In this PR, the changes that needed to delete an existing task from a general job have been added to TaskDriver.
  Also, the changes that needed on the controller side to drop the task from the instances and removing the task from the
  context has been implemented.

Tests

• The following tests are written for this issue:
  TestAddDeleteTask

• The following is the result of the "mvn test" command on the appropriate module:

[INFO] Tests run: 1242, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 5,182.218 s - in TestSuite
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1242, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  01:26 h
[INFO] Finished at: 2020-10-31T15:48:43-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[junkaixue]

Overall logic looks good.

[alirezazamani]

This PR is ready to be merged.

Final commit message:

Implement deleteTask API

In this commit, the changes needed to delete an existing
task from a general job have been added to TaskDriver.
Also, the changes that are needed on the controller side to
drop the task from the instances and remove the task from the
context have been implemented.