Fix sqlstatistics and clientconnectionid

[yukiwongky]

Port fixes from Microsoft.Data.SqlClient PR#341.

Before the change, the ConnectionTime is reported as MaxValue/1000 because the _closeTimestamp was not set properly. Further, calling RetrieveStatistics multiple times causes the ConnectionTime to increase exponentially because the total connection time was SafeAdded to the previously saves _connectionTime. The _connectionTime should be overwritten instead.

When the connection is closed, the ClientConnectionId is set to empty and it's unavailable to the user. The new change allows the user to access the ClientConnectionId even after the connection is closed.

[stephentoub]

Wouldn't this be better as

if ((reconnectTask != null && !reconnectTask.IsCompleted) || InnerConnection is DbConnectionClosedPreviouslyOpened)
{
   ...
}

? That way we're not doing the cast unless we have to, we're not adding an unnecessary local, etc.

[stephentoub]

The change this is porting added a test. Can we add the similar test here?

[stephentoub]

@yukiwongky? @cheenamalhotra?

[cheenamalhotra]

Hi @stephentoub I'll be creating a new PR soon to replace this one with comments addressed since Yuki is no longer working with SqlClient team.

[stephentoub]

Ah, ok, thanks for following up. I will close this then.