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.

lnwallet/chancloser/log.go

@@ -0,0 +1,41 @@
+package chancloser
+
+import (
+	"github.com/btcsuite/btclog"
+	"github.com/lightningnetwork/lnd/build"
+)
+
+// chancloserLog is a logger that is initialized with the btclog.Disabled
+// logger.
+var chancloserLog btclog.Logger
+
+// The default amount of logging is none.
+func init() {
+	UseLogger(build.NewSubLogger("CHCL", nil))
+}
+
+// DisableLog disables all logging output.
+func DisableLog() {
+	UseLogger(btclog.Disabled)
+}
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	chancloserLog = logger
+}
+
+// logClosure is used to provide a closure over expensive logging operations
+// so they aren't performed when the logging level doesn't warrant it.
+type logClosure func() string
+
+// String invokes the underlying function and returns the result.
+func (c logClosure) String() string {
+	return c()
+}
+
+// newLogClosure returns a new closure over a function that returns a string
+// which itself provides a Stringer interface so that it can be used with the
+// logging system.
+func newLogClosure(c func() string) logClosure {
+	return logClosure(c)
+}