Support rpc call cancellation/control via context.Context

[acabarbaye]

Currently actions executed in the plugin cannot be cancelled as the RPC client Call function does not support the use of context.

g.client.Call("Plugin.Greet", new(interface{}), &resp)

This could be problematic if the action takes a long time and we would like to control it via deadline/timeout
The only possible option seems to be kill the whole plugin server unless I missed something.

Would it be possible to leverage packages such as https://github.com/keegancsmith/rpc which propose similar clients to rpc.client but which support context.Context. This way we could use the context to cancel calls or to add a timeout protection