Fixes CDB_UserDataSize failing due ERROR: relation "*" does not exist.

Adds new _CDB_total_relation_size function that handles nonexistent
tables and does fallback to size=0.

That function could be used to cache total relation size or query another
table view with a cached total relation size.

Fixes #108

test/CDB_QuotaTest_expect

@@ -8,6 +8,9 @@ ERROR:  Quota exceeded by 3.9990234375KB
 INSERT 0 2048
 INSERT 0 2048
 INSERT 0 2048
+581632
+1163264
+0
 2
 ERROR:  Quota exceeded by 567.998046875KB
 0