fn: reduce lbagent and agent dependency (#938)

* fn: reduce lbagent and agent dependency

lbagent and agent code is too dependent. This causes
any changed in agent to break lbagent. In reality, for
LB there should be no delegated agent. Splitting these
two will cause some code duplication, but it reduces
dependency and complexity (eg. agent without docker)

* fn: post rebase fixup

* fn: runner/runnercall should use lbDeadline

* fn: fixup ln agent test

* fn: remove agent create option for common.WaitGroup

api/agent/func_logger.go

@@ -74,6 +74,20 @@ type nopCloser struct {
 
 func (n *nopCloser) Close() error { return nil }
 
+type nullReadWriter struct {
+	io.ReadCloser
+}
+
+func (n *nullReadWriter) Close() error {
+	return nil
+}
+func (n *nullReadWriter) Read(b []byte) (int, error) {
+	return 0, io.EOF
+}
+func (n *nullReadWriter) Write(b []byte) (int, error) {
+	return 0, io.EOF
+}
+
 // multiWriteCloser returns the first write or close that returns a non-nil
 // err, if no non-nil err is returned, then the returned bytes written will be
 // from the last call to write.