p2p: reduce log severity (#5338)

## Description

This PR aims to reduce the amount of `Logger.Error(..)` calls. Many of these calls are benign and do not need any intervention. 

Went from: 
```
node1    | E[2020-09-08|14:32:48.407] Connection failed @ recvRoutine (reading byte) module=p2p peer=af8747a81383f40583ae8790d2cc1f92cc7e4a35@192.167.10.4:26656 conn=MConn{192.167.10.4:26656} err="read tcp 192.167.10.3:48614->192.167.10.4:26656: read: connection reset by peer"
node1    | E[2020-09-08|14:32:48.407] Stopping peer for error                      module=p2p peer="Peer{MConn{192.167.10.4:26656} af8747a813 out}" err="read tcp 192.167.10.3:48614->192.167.10.4:26656: read: connection reset by peer"
node1    | E[2020-09-08|14:32:48.407] Error while stopping peer                    module=p2p peer=af8747a81383f40583ae8790d2cc1f92cc7e4a35@192.167.10.4:26656 err="already stopped"
node1    | E[2020-09-08|14:32:48.408] MConnection flush failed                     module=p2p peer=af8747a81383f40583ae8790d2cc1f92cc7e4a35@192.167.10.4:26656 err="write tcp 192.167.10.3:48614->192.167.10.4:26656: use of closed network connection"
```
To: 
```
node1    | E[2020-09-08|14:42:54.023] Stopping peer for error                      module=p2p peer="Peer{MConn{192.167.10.5:37844} e3d01d1795 in}" err=EOF
```

Closes: #4937

p2p/conn/connection.go

@@ -323,7 +323,7 @@ func (c *MConnection) flush() {
 	c.Logger.Debug("Flush", "conn", c)
 	err := c.bufConnWriter.Flush()
 	if err != nil {
-		c.Logger.Error("MConnection flush failed", "err", err)
+		c.Logger.Debug("MConnection flush failed", "err", err)
 	}
 }
 
@@ -597,7 +597,7 @@ FOR_LOOP:
 				if err == io.EOF {
 					c.Logger.Info("Connection is closed @ recvRoutine (likely by the other side)", "conn", c)
 				} else {
-					c.Logger.Error("Connection failed @ recvRoutine (reading byte)", "conn", c, "err", err)
+					c.Logger.Debug("Connection failed @ recvRoutine (reading byte)", "conn", c, "err", err)
 				}
 				c.stopForError(err)
 			}
@@ -626,7 +626,7 @@ FOR_LOOP:
 			channel, ok := c.channelsIdx[byte(pkt.PacketMsg.ChannelID)]
 			if !ok || channel == nil {
 				err := fmt.Errorf("unknown channel %X", pkt.PacketMsg.ChannelID)
-				c.Logger.Error("Connection failed @ recvRoutine", "conn", c, "err", err)
+				c.Logger.Debug("Connection failed @ recvRoutine", "conn", c, "err", err)
 				c.stopForError(err)
 				break FOR_LOOP
 			}
@@ -634,7 +634,7 @@ FOR_LOOP:
 			msgBytes, err := channel.recvPacketMsg(*pkt.PacketMsg)
 			if err != nil {
 				if c.IsRunning() {
-					c.Logger.Error("Connection failed @ recvRoutine", "conn", c, "err", err)
+					c.Logger.Debug("Connection failed @ recvRoutine", "conn", c, "err", err)
 					c.stopForError(err)
 				}
 				break FOR_LOOP

p2p/peer.go

@@ -201,7 +201,7 @@ func (p *peer) OnStop() {
 	p.metricsTicker.Stop()
 	p.BaseService.OnStop()
 	if err := p.mconn.Stop(); err != nil { // stop everything and close the conn
-		p.Logger.Error("Error while stopping peer", "err", err)
+		p.Logger.Debug("Error while stopping peer", "err", err)
 	}
 }