build: switch to slog Handlers

Start using the new slog handlers. With this commit we also remove the
need for the LogWriter since we let our LogRotator implement io.Writer
and pass that in to our log file handler.

build/log.go

@@ -1,9 +1,9 @@
 package build
 
 import (
-	"io"
+	"os"
 
-	"github.com/btcsuite/btclog"
+	"github.com/btcsuite/btclog/v2"
 )
 
 // LogType is an indicating the type of logging specified by the build flag.
@@ -60,17 +60,6 @@ func SuportedLogCompressor(logCompressor string) bool {
 	return ok
 }
 
-// LogWriter is a stub type whose behavior can be changed using the build flags
-// "stdlog" and "nolog". The default behavior is to write to both stdout and the
-// RotatorPipe. Passing "stdlog" will cause it only to write to stdout, and
-// "nolog" implements Write as a no-op.
-type LogWriter struct {
-	// RotatorPipe is the write-end pipe for writing to the log rotator.  It
-	// is written to by the Write method of the LogWriter type. This only
-	// needs to be set if neither the stdlog or nolog builds are set.
-	RotatorPipe *io.PipeWriter
-}
-
 // NewSubLogger constructs a new subsystem log from the current LogWriter
 // implementation. This is primarily intended for use with stdlog, as the actual
 // writer is shared amongst all instantiations.
@@ -104,8 +93,10 @@ func NewSubLogger(subsystem string,
 		// that they share the same backend, since all output is written
 		// to std out.
 		case LogTypeStdOut:
-			backend := btclog.NewBackend(&LogWriter{})
-			logger := backend.Logger(subsystem)
+			backend := btclog.NewDefaultHandler(os.Stdout)
+			logger := btclog.NewSLogger(
+				backend.SubSystem(subsystem),
+			)
 
 			// Set the logging level of the stdout logger to use the
 			// configured logging level specified by build flags.