Update ssb-db to 20.4.0

I encountered ssbc#734 and after installing [email protected] this stalling issue went away, so it appears to fix ssbc#734.

package.json

@@ -50,7 +50,7 @@
     "ssb-caps": "^1.0.1",
     "ssb-client": "^4.7.9",
     "ssb-config": "^3.2.5",
-    "ssb-db": "^20.0.1",
+    "ssb-db": "^20.4.0",
     "ssb-ebt": "^5.6.1",
     "ssb-friends": "^4.1.0",
     "ssb-gossip": "^1.1.0",