Add cache configuration (python#577)

master/master.cfg

@@ -110,6 +110,22 @@ 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,
+    "chdicts": 100,
+    "BuildRequests": 100,
+    "SourceStamps": 200,
+    "ssdicts": 200,
+    "objectids": 10,
+    "usdicts": 100,
+}
+
 # workers are set up in workers.py
 c["workers"] = [w.bb_worker for w in WORKERS]