Keep TCP session open

[cocowalla]

Currently a new TCP session is being opened for every message that is sent, which is really inefficient - the overhead of initiating and tearing down the connection likely exceeds that of sending the single TCP frame :)

Instead, a session should be opened only once, similar to how the original NLog sink handles it.

[vermeeca]

Thanks, and good suggestion. No reason I can see not to do this. I'll go ahead and slap an up-for-grabs label on this one if you (or anyone else) wants to do it before I get the chance to.