Added an option to add arbitrary headers to the client requests

The headers are key/value pairs defined in the settings under the `request.headers` namespace.

Also changed test infrastructure's so it'd be possible to define the additional settings for transport clients.
well... the point is that configuring nodes and configuring transport clients are two different things. Right now, the transport client configuration is randomized internally without the ability to modify it.. you might want to have your own custom settings for the client. The internal randomization is still there, but at least have the ability to override/force certain settings.