tka: keep the CompactionDefaults alongside the other limits

Updates #cleanup

Change-Id: Ib5e481d5a9c7ec7ac3e6b3913909ab1bf21d7a4d
Signed-off-by: Alex Chan <alexc@tailscale.com>

tka/limits.go

@@ -3,6 +3,10 @@
 
 package tka
 
+import (
+	"time"
+)
+
 const (
 	// Upper bound on checkpoint elements, chosen arbitrarily. Intended
 	// to cap the size of large AUMs.
@@ -22,3 +26,10 @@ const (
 	// Limit on scanning AUM trees, chosen arbitrarily.
 	maxScanIterations = 2000
 )
+
+var (
+	CompactionDefaults = CompactionOptions{
+		MinChain: 24,                  // Keep at minimum 24 AUMs since head.
+		MinAge:   14 * 24 * time.Hour, // Keep 2 weeks of AUMs.
+	}
+)