[FIX] service: error database in use on duplicate database

When trying to duplicate a database, while
having opened connections to it, postgresql throws

```
OperationalError: source database "duplicate_test"
is being accessed by other users
DETAIL:  There is 1 other session using the database.
```

Connections must be dropped before trying duplication
using TEMPLATE to avoid this.

openerp/service/db.py

@@ -97,6 +97,7 @@ def exp_duplicate_database(db_original_name, db_name):
     db = openerp.sql_db.db_connect('postgres')
     with closing(db.cursor()) as cr:
         cr.autocommit(True)     # avoid transaction block
+        _drop_conn(cr, db_original_name)
         cr.execute("""CREATE DATABASE "%s" ENCODING 'unicode' TEMPLATE "%s" """ % (db_name, db_original_name))
 
     from_fs = openerp.tools.config.filestore(db_original_name)