chancloser: new package for cooperative channel closure

Introduces a new chancloser package which exposes a ChanCloser
struct that handles the cooperative channel closure negotiation
and is meant to replace chancloser.go in the lnd package. Updates
all references to chancloser.go to instead use chancloser package.

test_utils.go

@@ -24,6 +24,7 @@ import (
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+	"github.com/lightningnetwork/lnd/lnwallet/chancloser"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
 	"github.com/lightningnetwork/lnd/shachain"
@@ -441,7 +442,7 @@ func createTestPeer(notifier chainntnfs.ChainNotifier, publTx chan *wire.MsgTx,
 		activeChannels: make(map[lnwire.ChannelID]*lnwallet.LightningChannel),
 		newChannels:    make(chan *newChannelMsg, 1),
 
-		activeChanCloses:   make(map[lnwire.ChannelID]*channelCloser),
+		activeChanCloses:   make(map[lnwire.ChannelID]*chancloser.ChanCloser),
 		localCloseChanReqs: make(chan *htlcswitch.ChanClose),
 		chanCloseMsgs:      make(chan *closeMsg),