implement simple job chaining

[pkit]

this patch adds another middleware ChainMiddleware the middleware allows rescheduling of response from previous job as a new job for the chain to work the following statements should be true:

• response should contain X-Zerovm-Execute: 1.0 header
• response should contain Content-Type: application/json header, and effectively be a json job description

you can see that the application that tries to chain something should be able to set a response header which means that application MUST use message/cgi or message/http as its output channel content type ChainMiddleware MUST be placed anywhere before ProxyQueryMiddleware in the proxy-server pipeline

example job description that can output a chained job is:

[
        {
            'name': 'http',
            'exec': {'path': 'swift://a/c/chainer'},
            'devices': [
                {
                    'name': 'stdout',
                    'content_type': 'message/http'
                }
            ]
        }
]

as you can see, stdout has no path - will be sent to user, and its content_type is message/http

Protection from infinite loops: ChainMiddleware has a config file setting chain_timeout (in seconds), by default chain_timeout = 20. If chain is executing more than chain_timeout the chain will stop and the resulting job description file will be returned to user.