p2p: set RecvMessageCapacity to maxMsgSize in all reactors

to prevent malicious nodes from sending us large messages (~21MB, which
is the default `RecvMessageCapacity`)

This allows us to remove unnecessary `maxMsgSize` check in `decodeMsg`. Since each channel has a msg capacity set to `maxMsgSize`, there's no need to check it again in `decodeMsg`.

Closes #1503

evidence/reactor.go

@@ -51,6 +51,7 @@ func (evR *Reactor) GetChannels() []*p2p.ChannelDescriptor {
 		{
 			ID:       EvidenceChannel,
 			Priority: 5,
+			RecvMessageCapacity: maxMsgSize,
 		},
 	}
 }
@@ -235,9 +236,6 @@ func RegisterMessages(cdc *amino.Codec) {
 }
 
 func decodeMsg(bz []byte) (msg Message, err error) {
-	if len(bz) > maxMsgSize {
-		return msg, fmt.Errorf("msg exceeds max size (%d > %d)", len(bz), maxMsgSize)
-	}
 	err = cdc.UnmarshalBinaryBare(bz, &msg)
 	return
 }