fix: Ensure rate-limits are on for get-entriesmain

Backoff on 4XX and 5XX.  See related issue:
https://github.com/google/certificate-transparency-go/issues/898

Test manually hints:

```
$ cat srv.py
from http.server import HTTPServer, BaseHTTPRequestHandler

class RequestHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(429)
        self.send_header("Content-Type", "text/plain")
        self.end_headers()
        self.wfile.write(b"429 something something...")

    def do_POST(self):
        self.do_GET()

    def do_PUT(self):
        self.do_GET()

    def do_DELETE(self):
        self.do_GET()

if __name__ == "__main__":
    server_address = ('localhost', 9090)
    httpd = HTTPServer(server_address, RequestHandler)
    print("Server running on http://localhost:9090")
    httpd.serve_forever()
```

And a transport for http.Client that redirects to localhost:

```
type statusRR struct {
	inner http.RoundTripper
}

func (s *statusRR) RoundTrip(req *http.Request) (*http.Response, error) {
	if strings.Contains(req.URL.Path, "ct/v1/get-entries") {
		req.URL.Scheme = "http"
		req.URL.Host = "localhost:9090"
	}
	rsp, err := s.inner.RoundTrip(req)
	return rsp, err
}
```

1 files changed, 2 insertions, 1 deletions

diff --git a/internal/monitor/monitor.go b/internal/monitor/monitor.go
index 1f068b2..2575977 100644
--- a/internal/monitor/monitor.go
+++ b/internal/monitor/monitor.go
@@ -173,7 +173,8 @@ func (mon *Monitor) newTailRFC6962(log MonitoredLog) (tail, error) {
 		return tail{}, err
 	}
 
-	return tail{cfg: mon.cfg, scanner: cli, checker: cli, matcher: mon.matcher}, nil
+	bc := &backoffClient{cli: cli}
+	return tail{cfg: mon.cfg, scanner: bc, checker: bc, matcher: mon.matcher}, nil
 }
 
 func (mon *Monitor) newTailTile(cfg MonitoredLog) (tail, error) {