Closescoder/internal#563

TheCoder Connect tunnel receives workspace state from the Coder server over adRPC stream. When first connecting to this stream, the current state of the user's workspaces is received, with subsequent messages being diffs on top of that state.

However, if the client disconnects from this stream, such as when the user's device is suspended, and then reconnects later, no mechanism exists for the tunnel to differentiate that message containing the entire initial state from another diff, and so that state is incorrectly applied as a diff.

In practice:

• Tunnel connects, receives a workspace update containing all the existing workspaces & agents.
• Tunnel loses connection, but isn't completely stopped.
• All the user's workspaces are restarted, producing a new set of agents.
• Tunnel regains connection, and receives a workspace update containing all the existing workspaces & agents.
• This initial update is incorrectly applied as a diff, with the Tunnel's state containing both the old & new agents.

This PR introduces a solution in which tunnelUpdater, when created, sends a FreshState flag with the WorkspaceUpdate type. This flag is handled in the vpn tunnel in the following fashion:

• Preserve existing Agents
• Remove current Agents in the tunnel that are not present in the WorkspaceUpdate
• Remove unreferenced Workspaces