Enable new SQL query engine

[dai-chen]

PLEASE DO NOT MERGE THIS REQUEST UNTIL READY

Issue #, if available: #1005

Description of changes:

1. Change new engine setting default value to true
2. Disable new engine for old IT task in build.gradle and do nothing for new IT task
3. Remove unnecessary setting change calls in new ITs.

Documentation:

1. Add new intro doc and update README: https://github.com/opendistro-for-elasticsearch/sql/blob/feature/sql-new-engine-enabled/docs/dev/NewSQLEngine.md
2. Change user manual to guide how to disable it: https://github.com/opendistro-for-elasticsearch/sql/blob/feature/sql-new-engine-enabled/docs/user/admin/settings.rst#opendistro-sql-engine-new-enabled

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #989 (c43d8af) into develop (43f41bd) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #989   +/-   ##
==========================================
  Coverage      99.87%   99.87%           
  Complexity      2406     2406           
==========================================
  Files            234      234           
  Lines           5524     5524           
  Branches         357      357           
==========================================
  Hits            5517     5517           
  Misses             5        5           
  Partials           2        2           

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[penghuo]

Thanks for the change.

[harold-wang]

Thanks for enabling new SQL engine.