raft: require app to consume result from Ready()

I changed `(*RawNode).Ready`'s behavior in #10892 in a problematic way.
Previously, `Ready()` would create and immediately "accept" a Ready
(i.e. commit the app to actually handling it). In #10892, Ready() became
a pure read-only operation and the "accepting" was moved to
`Advance(rd)`.  As a result it was illegal to use the RawNode in certain
ways while the Ready was being handled. Failure to do so would result in
dropped messages (and perhaps worse). For example, with the following
operations

1. `rd := rawNode.Ready()`
2. `rawNode.Step(someMsg)`
3. `rawNode.Advance(rd)`

`someMsg` would be dropped, because `Advance()` would clear out the
outgoing messages thinking that they had all been handled by the client.
I mistakenly assumed that this restriction had existed prior, but this
is incorrect.

I noticed this while trying to pick up the above PR in CockroachDB,
where it caused unit test failures, precisely due to the above example.

This PR reestablishes the previous behavior (result of `Ready()` must
be handled by the app) and adds a regression test.

While I was there, I carried out a few small clarifying refactors.

raft/node.go

@@ -316,7 +316,7 @@ func (n *node) run(rn *RawNode) {
 			// handled first, but it's generally good to emit larger Readys plus
 			// it simplifies testing (by emitting less frequently and more
 			// predictably).
-			rd = rn.Ready()
+			rd = rn.readyWithoutAccept()
 			readyc = n.readyc
 		}
 
@@ -387,7 +387,7 @@ func (n *node) run(rn *RawNode) {
 			rn.acceptReady(rd)
 			advancec = n.advancec
 		case <-advancec:
-			rn.commitReady(rd)
+			rn.Advance(rd)
 			rd = Ready{}
 			advancec = nil
 		case c := <-n.status: