Fix leaking ssh connections with VS Attach to Process

[WardenGnaw]

This PR address the issue where every SSH PortSupplier AD7Port will always create a new SSH connection on the remote machine when the Attach to Process window opens and closes. This causes ports to be kept open until VS completly closes.

The changes here are to cache an existing connection if VS opens and closes the attach to process window.
Adds a ref count mechanism to ensure if you are debugging multiple processes on the same machine, it does not Clean the connection until all processes are done.

Testing:
Ran ss -t state established sport = :22 to see how many connections exist.

• Open Attach to Process, Attach to C++ Process with GDB = One Connection
• Attach to Process and Detach = 0 Connections
• Attach, Detach, and Reattach = 1 Connection
• Attach, Detach, and Reattach and Detach = 0 Connections
• Attach x2 Processes = 2 Connections
• Attach x2, Detach All = 0 Connections
• Attach x2, Detach All, Reattach x2 = 2 Connections
• Attach -> Detach -> Attach - >Detach -> Attach -> Detach = 0 Connections
• Open Attach to Process and Closed Window = 0 Connections (after second commit)

[gregg-miskelly]

Otherwise LGTM