Add support for sessions to share a global threadpool. (#3177)
* Add support for sessions to share a global threadpool.
* Fix build issues
* Add tests, fix build issues.
* Added some documentation
* Fix centos issue when threadpools become nullptr due to 1 core.
* Fix mac and x86 build issues
* Address some PR comments
* Disabled test for android, added few more tests and addressed more PR comments.
* const_cast