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/rpc_client.go

@@ -188,8 +188,8 @@ func (c *Client) WaitForConnectivity(ctx context.Context) error {
 			data, dataErr := c.dataClient.ReqPing(ctx, &req)
 			// dataClient uses transport.Connecter, which doesn't expose WaitForReady(true)
 			// => we need to mask dial timeouts
-			if err, ok := dataErr.(interface{ Temporary() bool }); ok && err.Temporary() {
-				// Rate-limit pings here in case Temporary() is a mis-classification
+			if err, ok := dataErr.(interface{ Timeout() bool }); ok && err.Timeout() {
+				// Rate-limit pings here in case Timeout() == true is a mis-classification
 				// or returns immediately (this is a tight loop in that case)
 				// TODO keep this in lockstep with controlClient
 				// 		=> don't use FailFast for control, but check that both control and data worked