PLT-7407: Back-end plugins (#7409)

* tie back-end plugins together

* fix comment typo

* add tests and a bit of polish

* tests and polish

* add test, don't let backend executable paths escape the plugin directory

plugin/rpcplugin/ipc.go

@@ -19,7 +19,7 @@ func NewIPC() (io.ReadWriteCloser, []*os.File, error) {
 		childWriter.Close()
 		return nil, nil, err
 	}
-	return NewReadWriteCloser(parentReader, parentWriter), []*os.File{childReader, childWriter}, nil
+	return NewReadWriteCloser(NewAsyncReadCloser(parentReader), NewAsyncWriteCloser(parentWriter)), []*os.File{childReader, childWriter}, nil
 }
 
 // Returns the IPC instance inherited by the process from its parent.