Rework map session

This commit restructures the map session in to a struct
holding the state of what is needed during its lifetime.

For streaming sessions, the event loop is structured a
bit differently not hammering the clients with updates
but rather batching them over a short, configurable time
which should significantly improve cpu usage, and potentially
flakyness.

The use of Patch updates has been dialed back a little as
it does not look like its a 100% ready for prime time. Nodes
are now updated with full changes, except for a few things
like online status.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/mapper/tail_test.go

@@ -182,13 +182,16 @@ func TestTailNode(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			cfg := &types.Config{
+				BaseDomain:          tt.baseDomain,
+				DNSConfig:           tt.dnsConfig,
+				RandomizeClientPort: false,
+			}
 			got, err := tailNode(
 				tt.node,
 				0,
 				tt.pol,
-				tt.dnsConfig,
-				tt.baseDomain,
-				false,
+				cfg,
 			)
 
 			if (err != nil) != tt.wantErr {