Avoid tables name clashing when executing within same transaction

Generate more unique temp table names when the CDB_SyncTable function
is executed multiple times within the same transaction.

When executed in isolation, there will be always an implicit
surrounding transaction.

But when executed several times within the same transaction it can
give an `ERROR:  relation "src_sync_718794" already exists`.

E.g:

```
BEGIN;
SELECT cartodb.CDB_SyncTable('source1', 'public', 'dest1');
SELECT cartodb.CDB_SyncTable('source12, 'public', 'dest2');
COMMIT;
```

scripts-available/CDB_SyncTable.sql

@@ -52,6 +52,22 @@ BEGIN
 END;
 $$ LANGUAGE plpgsql IMMUTABLE PARALLEL SAFE;
 
+
+/*
+    Given a prefix, generate a safe unique NAME for a temp table.
+
+    Example of usage:
+
+       SELECT __CDB_GenerateUniqueName('src_sync'); --> src_sync_718794_120106
+
+*/
+CREATE OR REPLACE FUNCTION __CDB_GenerateUniqueName(prefix TEXT)
+RETURNS NAME
+AS $$
+  SELECT format('%s_%s_%s', prefix, txid_current(), (random()*1000000)::int)::NAME;
+$$ LANGUAGE sql VOLATILE PARALLEL UNSAFE;
+
+
 /*
    A Table Syncer
 
@@ -94,8 +110,8 @@ BEGIN
   SELECT ARRAY(SELECT quote_ident(c) FROM _CDB_GetColumns(src_table) as c WHERE c <> 'cartodb_id') INTO colnames;
   quoted_colnames := array_to_string(colnames, ',');
 
-  src_hash_table_name := format('src_sync_%s', txid_current());
-  dst_hash_table_name := format('dst_sync_%s', txid_current());
+  src_hash_table_name := __CDB_GenerateUniqueName('src_sync');
+  dst_hash_table_name := __CDB_GenerateUniqueName('dst_sync');
 
   EXECUTE format('CREATE TEMP TABLE %I(cartodb_id BIGINT, hash TEXT) ON COMMIT DROP', src_hash_table_name);
   EXECUTE format('CREATE TEMP TABLE %I(cartodb_id BIGINT, hash TEXT) ON COMMIT DROP', dst_hash_table_name);