fixup! Add cache configuration

master/master.cfg

@@ -110,6 +110,11 @@ c["configurators"] = [
     )
 ]
 
+# Note: these cache values are not currently tuned in any meaningful way.
+# Some are taken straight from the buildbot docs at
+# https://docs.buildbot.net/4.2.1/manual/configuration/global.html#caches
+# and others are just guesses.  For now, they're mostly meant to see if
+# there's any appreciable impact on performance or memory usage.
 c["caches"] = {
     "Changes": 100,
     "Builds": 500,