SQL Server improvements to asynchronous code (#502)

Improve some areas of the SqlServer memory connector:
* avoid I/O in ctor, which could cause the service to crash with
dependency injection errors. Run the initialization logic on the first
call to any public method instead.
* change methods using I/O from sync to async
* use async dispose for connection, command, and data reader instances
* add missing ConfigureAwait(false) on instances async disposing logic