suggestExitNodeLocked now ranks exit node candidates using the per-region
latency tracked by the netcheck Client (RecentRegionLatency), which merges
the reports retained in c.prev. That history is only useful for far-away
regions if it contains a full netcheck report, since incremental reports
only re-probe the home region and a handful of the fastest ones.
The full-report cadence in GetReport and the c.prev retention window were
two independent 5-min constants - the way we schedule netchecks ensured
that the history always contaned a full report, but it was not a strong
contract and we did not have any checks around this.
Now full report interval and retention window are driven by the same
var, and a test confirms that the history contains a full report.
Updates tailscale/corp#17516
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
e9e209673e