This contains the preparatory work for the merge of SqlInternalConnectionTds. We're working with a smaller code file here, so I've merged every part of the two files which avoids a behavioural change.

This should be easier to review comment-by-comment - it's bulky, but not complex. I'd also appreciate a CI run.

At the end of this, the diff is +43/-80. Remaining differences here are:

• Optimisation: netcore will clear the array of session state when the connection resets, netfx will replace it with a new array.
• netfx will consider four extra error codes from the server to be transient: 40020, 40143, 40166, 40540.
• netfx contains a little context connection support.
• netfx contains support for Transparent Network IP Resolution, while netcore does not.
• netfx supports the Network Library connection string parameter.
• netcore will throw an exception if it receives an ENV_LOGSHIPNODE ENVCHANGE when the connection string's Application Intent is read-only, while netfx will not.
• When getting the federated authentication token, netfx will stop retrying if the next pause between each attempt to acquire a token would cause the connection timeout to be expired.
• netcore supports passing the User ID parameter to MSAL as an initial username; netfx does not.
• Some exception messages are more precise in netfx than they are in netcore.

What should we do with Transparent Network IP Resolution? This functionality doesn't exist in netcore at all - does it need to be ported? If it only exists in netfx for backwards compatibility, should it be made obsolete?

I'd also be interested to know what the right style of exception messages is. If (for example) the connection receives an unexpected session recovery feature extension acknowledgement, which exception message should I merge in favour of?

Internal connection fatal error. Error state: {ParsingErrorState}, Feature Id: {ID of feature}.

Internal connection fatal error.

The former is netfx, the latter is netcore. I prefer the detail of netfx, but I imagine netcore changed this for a reason.