rpc: /dial_peers: only mark peers as persistent if flag is on (#3620)

## Description

also

    handle errors from DialPeersAsync
    remove nil addr from log msg
    fix TestPEXReactorDoesNotDisconnectFromPersistentPeerInSeedMode

This is a follow-up from
#3593 (review)

Fixes most of the #3617, except #3593 (comment)

## Commits

* rpc: /dial_peers: only mark peers as persistent if flag is on

also

- handle errors from DialPeersAsync
- remove nil addr from log msg
- fix TestPEXReactorDoesNotDisconnectFromPersistentPeerInSeedMode

This is a follow-up from
https://github.com/tendermint/tendermint/pull/3593#pullrequestreview-233556909

* remove a call to AddPersistentPeers

TestDialFail will trigger a reconnect

p2p/switch_test.go

@@ -410,7 +410,6 @@ func TestSwitchReconnectsToOutboundPersistentPeer(t *testing.T) {
 
 	err = sw.DialPeerWithAddress(rp.Addr())
 	require.Nil(t, err)
-	time.Sleep(50 * time.Millisecond)
 	require.NotNil(t, sw.Peers().Get(rp.ID()))
 
 	p := sw.Peers().List()[0]
@@ -432,7 +431,7 @@ func TestSwitchReconnectsToOutboundPersistentPeer(t *testing.T) {
 	defer rp.Stop()
 
 	conf := config.DefaultP2PConfig()
-	conf.TestDialFail = true
+	conf.TestDialFail = true // will trigger a reconnect
 	err = sw.addOutboundPeerWithConfig(rp.Addr(), conf)
 	require.NotNil(t, err)
 	// DialPeerWithAddres - sw.peerConfig resets the dialer