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/lib/pq/conn_go18.go

@@ -1,128 +0,0 @@
-// +build go1.8
-
-package pq
-
-import (
-	"context"
-	"database/sql"
-	"database/sql/driver"
-	"fmt"
-	"io"
-	"io/ioutil"
-)
-
-// Implement the "QueryerContext" interface
-func (cn *conn) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Rows, error) {
-	list := make([]driver.Value, len(args))
-	for i, nv := range args {
-		list[i] = nv.Value
-	}
-	finish := cn.watchCancel(ctx)
-	r, err := cn.query(query, list)
-	if err != nil {
-		if finish != nil {
-			finish()
-		}
-		return nil, err
-	}
-	r.finish = finish
-	return r, nil
-}
-
-// Implement the "ExecerContext" interface
-func (cn *conn) ExecContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
-	list := make([]driver.Value, len(args))
-	for i, nv := range args {
-		list[i] = nv.Value
-	}
-
-	if finish := cn.watchCancel(ctx); finish != nil {
-		defer finish()
-	}
-
-	return cn.Exec(query, list)
-}
-
-// Implement the "ConnBeginTx" interface
-func (cn *conn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
-	var mode string
-
-	switch sql.IsolationLevel(opts.Isolation) {
-	case sql.LevelDefault:
-		// Don't touch mode: use the server's default
-	case sql.LevelReadUncommitted:
-		mode = " ISOLATION LEVEL READ UNCOMMITTED"
-	case sql.LevelReadCommitted:
-		mode = " ISOLATION LEVEL READ COMMITTED"
-	case sql.LevelRepeatableRead:
-		mode = " ISOLATION LEVEL REPEATABLE READ"
-	case sql.LevelSerializable:
-		mode = " ISOLATION LEVEL SERIALIZABLE"
-	default:
-		return nil, fmt.Errorf("pq: isolation level not supported: %d", opts.Isolation)
-	}
-
-	if opts.ReadOnly {
-		mode += " READ ONLY"
-	} else {
-		mode += " READ WRITE"
-	}
-
-	tx, err := cn.begin(mode)
-	if err != nil {
-		return nil, err
-	}
-	cn.txnFinish = cn.watchCancel(ctx)
-	return tx, nil
-}
-
-func (cn *conn) watchCancel(ctx context.Context) func() {
-	if done := ctx.Done(); done != nil {
-		finished := make(chan struct{})
-		go func() {
-			select {
-			case <-done:
-				_ = cn.cancel()
-				finished <- struct{}{}
-			case <-finished:
-			}
-		}()
-		return func() {
-			select {
-			case <-finished:
-			case finished <- struct{}{}:
-			}
-		}
-	}
-	return nil
-}
-
-func (cn *conn) cancel() error {
-	c, err := dial(cn.dialer, cn.opts)
-	if err != nil {
-		return err
-	}
-	defer c.Close()
-
-	{
-		can := conn{
-			c: c,
-		}
-		can.ssl(cn.opts)
-
-		w := can.writeBuf(0)
-		w.int32(80877102) // cancel request code
-		w.int32(cn.processID)
-		w.int32(cn.secretKey)
-
-		if err := can.sendStartupPacket(w); err != nil {
-			return err
-		}
-	}
-
-	// Read until EOF to ensure that the server received the cancel.
-	{
-		_, err := io.Copy(ioutil.Discard, c)
-		return err
-	}
-}