* Adding ICompletionExtension interface
* Adding extension loading and execution logic
* Fixing compilation error in VS 2017
* Using MEF for completion extension discovery
* using await on GetCompletionItems
* Adding an integration test for completion extension and update the completion extension interface
* Update the completion extension test
* Fix issues based on review comments
* Remove try/cache based on review comments, fix a integration test.
* More changes based on review comments
* Fixing SendResult logic for completion extension loading
* Only load completion extension from the assembly passed in, add more comments in the test
* Adding right assert messages in the test.
* More fixes based on review comments
* Dropping ICompletionExtensionProvider, load assembly only if it's loaded at the first time or updated since last load.
* Fix based on the latest review comments
* Adding missing TSQL functions in default completion list
* Update jsonrpc documentation for completion/extLoad
