Try to retain existing cartodb_id values

Also reset sequence when original cartodb_id values correctly
cast to integers, and does not keep a backup in that case.

Closes #27

test/CDB_CartodbfyTableTest_expect

@@ -29,5 +29,13 @@ SELECT 1
 text timestamps cartodbfied fine
 @ 0|@ 0
 DROP TABLE
+SELECT 1
+text cartodb_id cartodbfied fine
+5
+DROP TABLE
+SELECT 1
+uncasting text cartodb_id cartodbfied fine
+1|nan
+DROP TABLE
 DROP FUNCTION
 DROP FUNCTION