cmd/stdiscosrv: New discovery server (fixes #4618)

This is a new revision of the discovery server. Relevant changes and
non-changes:

- Protocol towards clients is unchanged.

- Recommended large scale design is still to be deployed nehind nginx (I
  tested, and it's still a lot faster at terminating TLS).

- Database backend is leveldb again, only. It scales enough, is easy to
  setup, and we don't need any backend to take care of.

- Server supports replication. This is a simple TCP channel - protect it
  with a firewall when deploying over the internet. (We deploy this within
  the same datacenter, and with firewall.) Any incoming client announces
  are sent over the replication channel(s) to other peer discosrvs.
  Incoming replication changes are applied to the database as if they came
  from clients, but without the TLS/certificate overhead.

- Metrics are exposed using the prometheus library, when enabled.

- The database values and replication protocol is protobuf, because JSON
  was quite CPU intensive when I tried that and benchmarked it.

- The "Retry-After" value for failed lookups gets slowly increased from
  a default of 120 seconds, by 5 seconds for each failed lookup,
  independently by each discosrv. This lowers the query load over time for
  clients that are never seen. The Retry-After maxes out at 3600 after a
  couple of weeks of this increase. The number of failed lookups is
  stored in the database, now and then (avoiding making each lookup a
  database put).

All in all this means clients can be pointed towards a cluster using
just multiple A / AAAA records to gain both load sharing and redundancy
(if one is down, clients will talk to the remaining ones).

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4648

vendor/github.com/a8m/mark/cmd/mark/main.go

@@ -0,0 +1,94 @@
+// mark command line tool. available at https://github.com/a8m/mark
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/a8m/mark"
+)
+
+var (
+	input     = flag.String("i", "", "")
+	output    = flag.String("o", "", "")
+	smarty    = flag.Bool("smartypants", false, "")
+	fractions = flag.Bool("fractions", false, "")
+)
+
+var usage = `Usage: mark [options...] <input>
+
+Options:
+  -i  Specify file input, otherwise use last argument as input file. 
+      If no input file is specified, read from stdin.
+  -o  Specify file output. If none is specified, write to stdout.
+
+  -smartypants  Use "smart" typograhic punctuation for things like 
+                quotes and dashes.
+  -fractions    Traslate fraction like to suitable HTML elements
+`
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprint(os.Stderr, fmt.Sprintf(usage))
+	}
+	flag.Parse()
+	// read
+	var reader *bufio.Reader
+	if *input != "" {
+		file, err := os.Open(*input)
+		if err != nil {
+			usageAndExit(fmt.Sprintf("Error to open file input: %s.", *input))
+		}
+		defer file.Close()
+		reader = bufio.NewReader(file)
+	} else {
+		stat, err := os.Stdin.Stat()
+		if err != nil || (stat.Mode()&os.ModeCharDevice) != 0 {
+			usageAndExit("")
+		}
+		reader = bufio.NewReader(os.Stdin)
+	}
+	// collect data
+	var data string
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+			usageAndExit("failed to reading input.")
+		}
+		data += line
+	}
+	// write
+	var (
+		err  error
+		file = os.Stdout
+	)
+	if *output != "" {
+		if file, err = os.Create(*output); err != nil {
+			usageAndExit("error to create the wanted output file.")
+		}
+	}
+	// mark rendering
+	opts := mark.DefaultOptions()
+	opts.Smartypants = *smarty
+	opts.Fractions = *fractions
+	m := mark.New(data, opts)
+	if _, err := file.WriteString(m.Render()); err != nil {
+		usageAndExit(fmt.Sprintf("error writing output to: %s.", file.Name()))
+	}
+}
+
+func usageAndExit(msg string) {
+	if msg != "" {
+		fmt.Fprintf(os.Stderr, msg)
+		fmt.Fprintf(os.Stderr, "\n\n")
+	}
+	flag.Usage()
+	fmt.Fprintf(os.Stderr, "\n")
+	os.Exit(1)
+}