healthcheck: monitor access to chain backend

Add a new health check package which will periodically poll health
check functions and shutdown if we do not succeed after our set number
of attempts. The first check that we add is one for our chain backend,
to ensure that we are connected to a bitcoin node.

log.go

@@ -16,6 +16,7 @@ import (
 	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
+	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/lnrpc/autopilotrpc"
@@ -129,6 +130,7 @@ func SetupLoggers(root *build.RotatingLogWriter) {
 	AddSubLogger(root, routerrpc.Subsystem, routerrpc.UseLogger)
 	AddSubLogger(root, chanfitness.Subsystem, chanfitness.UseLogger)
 	AddSubLogger(root, verrpc.Subsystem, verrpc.UseLogger)
+	AddSubLogger(root, healthcheck.Subsystem, healthcheck.UseLogger)
 }
 
 // AddSubLogger is a helper method to conveniently create and register the