[INCOMPLETE] Plugin talker sync send

This pull request adds synchronous FCP send function for plugins running in the same node.

The codepath for plugins which are NOT running in the same node is NOT implemented. Therefore this pull-request is incomplete. 
I am sending it already nevertheless because WOT master branch needs this to compile. 
This code is almost entirely duplicated in PluginReplySenderDirect's implementation of the same method. Can this share more code?