tka: consolidate all the limits into a single file

This makes the limits easier to find and change, rather than scattering
them across the TKA code.

Updates #cleanup

Change-Id: I2f9b3b83d293eebb2572fa7bb6de2ca1f3d9a192
Signed-off-by: Alex Chan <alexc@tailscale.com>

tka/limits.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tka
+
+const (
+	// Upper bound on checkpoint elements, chosen arbitrarily. Intended
+	// to cap the size of large AUMs.
+	maxDisablementSecrets = 32
+	maxKeys               = 512
+
+	// Max amount of metadata that can be associated with a key, chosen arbitrarily.
+	// Intended to avoid people abusing TKA as a key-value score.
+	maxMetaBytes = 512
+
+	// Max iterations searching for any intersection during the sync process.
+	maxSyncIter = 2000
+
+	// Max iterations searching for a head intersection during the sync process.
+	maxSyncHeadIntersectionIter = 400
+
+	// Limit on scanning AUM trees, chosen arbitrarily.
+	maxScanIterations = 2000
+)