All the language clients support a special ignore parameter that doesn't get passed to elasticsearch with the request, but used to indicate which error code should not lead to an exception if returned for a specific request.
Moving this to the low level REST client will allow the high level REST client to make use of it too, for instance so that it doesn't have to intercept ResponseExceptions when the get api returns a 404.
This enables the RestClient to send array-based (multi-valued) header values, rather than only sending whatever happened to be the _last_ value of the header.
Some Rest tests use the Sun HTTP server which has lingering threads after shutdown. Similar to ESTestCase, this adds an
option to wait 5 seconds for these threads to terminate.
:client ---------> :client:rest
:client-sniffer -> :client:sniffer
:client-test ----> :client:test
This lines the client up with how we do things like modules and
plugins.
