Adding keepalives to the bitten client/server protocol. Keepalives now keep builds running as long as the slave is up, rather than having them time out independently of if the slave is working on them. This requires an update of both the master and slave to support keepalives-- older slaves (protocol version 3) will continue to work, but will not submit keepalives (and thus will retain previous behavior.) I've updated the PROTOCOL_VERSION field as older masters would kill builds for newer slaves that attempted to keepalive. We probably want to update test_create_build_protocol_wrong_version to test the matrix of allowed versions, but I expect the exception of allowing of older slaves to disappear when we next edit the protocol. Closes #411.

# -*- coding: utf-8 -*-
#
# Copyright (C) 2007 Edgewall Software
# Copyright (C) 2005-2007 Christopher Lenz <cmlenz@gmx.de>
# All rights reserved.
#
# This software is licensed as described in the file COPYING, which
# you should have received as part of this distribution. The terms
# are also available at http://bitten.edgewall.org/wiki/License.

__docformat__ = 'restructuredtext en'
try:
    __version__ = __import__('pkg_resources').get_distribution('Bitten').version
except:
    try:
        __version__ = __import__('pkg_resources').get_distribution(
                                                    'BittenSlave').version
    except:
        pass

# The master-slave protocol/configuration version
PROTOCOL_VERSION = 4