SQL Server improvements to asynchronous code

[dluc]

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

[dluc]

/cc @kbeaugrand