PR tailscale/tailscale#20646 changed the representation of DERP region
IDs from plain ints to a tailcfg.DERPRegionID type. This patch
requires that version of the tailscale.com library and updates our
code to also use the new type.
Updates: tailscale/tailscale#20165
(cherry picked from commit 79695ab51b)
TestInitialMapNotStarvedByReconnectStorm reproduces the #3346 stall;
TestPolicyManagerConcurrentReads guards the RLock cache access under -race.
Updates #3346