PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/rpcplugin/io.go

@@ -0,0 +1,23 @@
+package rpcplugin
+
+import (
+	"io"
+)
+
+type rwc struct {
+	io.ReadCloser
+	io.WriteCloser
+}
+
+func (rwc *rwc) Close() error {
+	rerr := rwc.ReadCloser.Close()
+	werr := rwc.WriteCloser.Close()
+	if rerr != nil {
+		return rerr
+	}
+	return werr
+}
+
+func NewReadWriteCloser(r io.ReadCloser, w io.WriteCloser) io.ReadWriteCloser {
+	return &rwc{r, w}
+}