multi: use uptime api for bitcoind healthcheck if version > 0.15

The getblockchaininfo call in bitcoind uses a commonly used lock,
csmain, in bitcoind. This made the endpoint unsuitable for a health
check, because some nodes were seeing waits up to 5 minutes (!). This
commit updates our health check function to use the uptime api, provided
our bitcoind version is > 0.15, when the api was added. We do not need
to switch our health check for btcd, because it has more granular
locking.

server.go

@@ -1292,10 +1292,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	// will not run it.
 	chainHealthCheck := healthcheck.NewObservation(
 		"chain backend",
-		func() error {
-			_, _, err := cc.ChainIO.GetBestBlock()
-			return err
-		},
+		cc.HealthCheck,
 		cfg.HealthChecks.ChainCheck.Interval,
 		cfg.HealthChecks.ChainCheck.Timeout,
 		cfg.HealthChecks.ChainCheck.Backoff,