chainio: introduce chainio to handle block synchronization

This commit inits the package `chainio` and defines the interface
`Blockbeat` and `Consumer`. The `Consumer` must be implemented by other
subsystems if it requires block epoch subscription.

chainio/log.go

@@ -0,0 +1,32 @@
+package chainio
+
+import (
+	"github.com/btcsuite/btclog/v2"
+	"github.com/lightningnetwork/lnd/build"
+)
+
+// Subsystem defines the logging code for this subsystem.
+const Subsystem = "CHIO"
+
+// clog is a logger that is initialized with no output filters. This means the
+// package will not perform any logging by default until the caller requests
+// it.
+var clog btclog.Logger
+
+// The default amount of logging is none.
+func init() {
+	UseLogger(build.NewSubLogger(Subsystem, nil))
+}
+
+// DisableLog disables all library log output. Logging output is disabled by
+// default until UseLogger is called.
+func DisableLog() {
+	UseLogger(btclog.Disabled)
+}
+
+// UseLogger uses a specified Logger to output package logging info. This
+// should be used in preference to SetLogWriter if the caller is also using
+// btclog.
+func UseLogger(logger btclog.Logger) {
+	clog = logger
+}