derp: intern key.NodePublic across the server

Consistently interning the NodePublic's throughout DERP, particularly
inside the maps reduces memory usage and reduces lookup costs in the
associated data structures.

It is not clear exactly how efficient the weak pointers will be in
practice, but estimating this using derpstress with 10k conns pushing
40kpps in each direction, this is patch grows heap at approximately half
the rate vs.  the old code and has fewer instances of long stalls that
trigger i/o timeouts for the clients.

Updates tailscale/corp#24485

Signed-off-by: James Tucker <james@tailscale.com>

types/key/node.go

@@ -10,6 +10,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"unique"
 
 	"go4.org/mem"
 	"golang.org/x/crypto/curve25519"
@@ -174,6 +175,16 @@ func (p NodePublic) Compare(p2 NodePublic) int {
 	return bytes.Compare(p.k[:], p2.k[:])
 }
 
+// Handle returns a unique.Handle for this NodePublic. The Handle is more
+// efficient for storage and comparison than the NodePublic itself, but is also
+// more expensive to create. It is best to keep a copy of the Handle on a longer
+// term object representing a NodePublic, rather than creating it on the fly,
+// but in doing so if the Handle is used in multiple other data structures the
+// cost of Handle storage and comparisons on lookups will quickly amortize.
+func (p NodePublic) Handle() unique.Handle[NodePublic] {
+	return unique.Make(p)
+}
+
 // ParseNodePublicUntyped parses an untyped 64-character hex value
 // as a NodePublic.
 //