go1.18: address net.Error.Temporary() deprecation

Go 1.18 deprecated net.Error.Temporary().
This commit cleans up places where we use it incorrectly.
Also, the rpc layer defines some errors that implement

  interface { Temporary() bool }

I added comments to all of the implementations to indicate
whether they will be required if net.Error.Temporary is ever
ever removed in the future.

For HandshakeError, the Temporary() return value is actually
important. I moved & rewrote a (previously misplaced) comment
there.

The ReadStreamError changes were
1. necessary to pacify newer staticcheck and
2. technically, an error can implement Temporary()
   without being net.Err. This applies to some syscall
   errors in the standard library.

Reading list for those interested:
- https://github.com/golang/go/issues/45729
- https://groups.google.com/g/golang-nuts/c/-JcZzOkyqYI
- https://man7.org/linux/man-pages/man2/accept.2.html

Note: This change was prompted by staticheck:

> SA1019: neterr.Temporary has been deprecated since Go 1.18 because it
> shouldn't be used: Temporary errors are not well-defined. Most
> "temporary" errors are timeouts, and the few exceptions are surprising.
> Do not use this method. (staticcheck)

rpc/dataconn/stream/stream.go

@@ -181,23 +181,19 @@ func (e *ReadStreamError) Error() string {
 
 var _ net.Error = &ReadStreamError{}
 
-func (e ReadStreamError) netErr() net.Error {
-	if netErr, ok := e.Err.(net.Error); ok {
-		return netErr
-	}
-	return nil
-}
-
 func (e ReadStreamError) Timeout() bool {
-	if netErr := e.netErr(); netErr != nil {
+	if netErr, ok := e.Err.(net.Error); ok {
 		return netErr.Timeout()
 	}
 	return false
 }
 
+// This function is deprecated in net.Error and since this
+// function is not involved in .Accept() code path, nothing
+// really needs this method to be here.
 func (e ReadStreamError) Temporary() bool {
-	if netErr := e.netErr(); netErr != nil {
-		return netErr.Temporary()
+	if te, ok := e.Err.(interface{ Temporary() bool }); ok {
+		return te.Temporary()
 	}
 	return false
 }

rpc/dataconn/stream/stream_conn.go

@@ -232,7 +232,12 @@ var _ net.Error = (*closeStateErrConnectionClosed)(nil)
 func (e *closeStateErrConnectionClosed) Error() string {
 	return "connection closed"
 }
-func (e *closeStateErrConnectionClosed) Timeout() bool   { return false }
+
+func (e *closeStateErrConnectionClosed) Timeout() bool { return false }
+
+// This function is deprecated in net.Error and since this
+// function is not involved in .Accept() code path, nothing
+// really needs this method to be here.
 func (e *closeStateErrConnectionClosed) Temporary() bool { return false }
 
 func (s *closeState) CloseEntry() error {