raft: fix bug in unbounded log growth prevention mechanism

The previous code was using the proto-generated `Size()` method to
track the size of an incoming proposal at the leader. This includes
the Index and Term, which were mutated after the call to `Size()`
when appending to the log. Additionally, it was not taking into
account that an ignored configuration change would ignore the
original proposal and append an empty entry instead.

As a result, a fully committed Raft group could end up with a non-
zero tracked uncommitted Raft log counter that would eventually hit
the ceiling and drop all future proposals indiscriminately. It would
also immediately imply that proposals exceeding the threshold alone
would get refused (as the "first uncommitted proposal" gets special
treatment and is always allowed in).

Track only the size of the payload actually appended to the Raft log
instead.

For context, see:
https://github.com/cockroachdb/cockroach/issues/31618#issuecomment-431374938

raft/node_test.go

@@ -1006,7 +1006,7 @@ func TestNodeBoundedLogGrowthWithPartition(t *testing.T) {
 	const maxEntries = 16
 	data := []byte("testdata")
 	testEntry := raftpb.Entry{Data: data}
-	maxEntrySize := uint64(maxEntries * testEntry.Size())
+	maxEntrySize := uint64(maxEntries * PayloadSize(testEntry))
 
 	s := NewMemoryStorage()
 	cfg := newTestConfig(1, []uint64{1}, 10, 1, s)