node: allow registration of custom reactors while creating node (#3771)

* change invocation of NewNode across

* custom reactor name are prefixed with CUSTOM_

* upgate changelog pending

* improve comments

* node: refactor NewNode to use functional options

p2p/mock/reactor.go

@@ -0,0 +1,23 @@
+package mock
+
+import (
+	"github.com/tendermint/tendermint/libs/log"
+	"github.com/tendermint/tendermint/p2p"
+	"github.com/tendermint/tendermint/p2p/conn"
+)
+
+type Reactor struct {
+	p2p.BaseReactor
+}
+
+func NewReactor() *Reactor {
+	r := &Reactor{}
+	r.BaseReactor = *p2p.NewBaseReactor("Reactor", r)
+	r.SetLogger(log.TestingLogger())
+	return r
+}
+
+func (r *Reactor) GetChannels() []*conn.ChannelDescriptor            { return []*conn.ChannelDescriptor{} }
+func (r *Reactor) AddPeer(peer p2p.Peer)                             {}
+func (r *Reactor) RemovePeer(peer p2p.Peer, reason interface{})      {}
+func (r *Reactor) Receive(chID byte, peer p2p.Peer, msgBytes []byte) {}