It's required to have more than one connection to the database for the
maxconnections configuration option to work with threads. However,
connection.close() is closing all the connections. Only close the connection
when no more thread need it.
Github-fix: https://github.com/OfflineIMAP/offlineimap/issues/350
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>