drain: fix receivable and 3ksat margin workaround

drain/drain.py

@@ -97,7 +97,10 @@ def spendable_from_scid(plugin, payload, scid=None):
         their_reserve = their
 
     spendable = channel_peer['spendable_msat']
-    receivable = their - their_reserve
+    receivable = channel_peer['receivable_msat']
+    if not receivable:
+        # receivable_msat was added with the 0.8.2 release, have a fallback
+        receivable = their - their_reserve - Millisatoshi('3000sat')
     return spendable, receivable