The ReturnMessageQueue/Rowsq implementation needs tweaks to message ordering

[shueybubbles]

go-mssqldb/messages_example_test.go

Line 46 in e538731

active := true

The message queue example illustrates that the application waits for the first MsgNext event before calling Rows.Next.

Today, ProcessQueryResponse doesn't post a MsgNext event until the reader encounters column metadata. However, the reader channel only has a 5 token buffer, so if there are 5 tokens in the queue before it reaches any columns, the query blocks. Posting MsgNext right away will nudge the app to read tokens in the channel.

Exposed by a sqlcmd issue microsoft/go-sqlcmd#48

[shueybubbles]

My earlier assertion about MsgNext isn't quite right.
We need to send MsgNextResult for every type of doneStruct and MsgNext for every column set.
We're also missing handling of ServerError and doneInProcStruct

This fix will also address microsoft/go-sqlcmd#47