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/rows.go

@@ -1,93 +0,0 @@
-package pq
-
-import (
-	"math"
-	"reflect"
-	"time"
-
-	"github.com/lib/pq/oid"
-)
-
-const headerSize = 4
-
-type fieldDesc struct {
-	// The object ID of the data type.
-	OID oid.Oid
-	// The data type size (see pg_type.typlen).
-	// Note that negative values denote variable-width types.
-	Len int
-	// The type modifier (see pg_attribute.atttypmod).
-	// The meaning of the modifier is type-specific.
-	Mod int
-}
-
-func (fd fieldDesc) Type() reflect.Type {
-	switch fd.OID {
-	case oid.T_int8:
-		return reflect.TypeOf(int64(0))
-	case oid.T_int4:
-		return reflect.TypeOf(int32(0))
-	case oid.T_int2:
-		return reflect.TypeOf(int16(0))
-	case oid.T_varchar, oid.T_text:
-		return reflect.TypeOf("")
-	case oid.T_bool:
-		return reflect.TypeOf(false)
-	case oid.T_date, oid.T_time, oid.T_timetz, oid.T_timestamp, oid.T_timestamptz:
-		return reflect.TypeOf(time.Time{})
-	case oid.T_bytea:
-		return reflect.TypeOf([]byte(nil))
-	default:
-		return reflect.TypeOf(new(interface{})).Elem()
-	}
-}
-
-func (fd fieldDesc) Name() string {
-	return oid.TypeName[fd.OID]
-}
-
-func (fd fieldDesc) Length() (length int64, ok bool) {
-	switch fd.OID {
-	case oid.T_text, oid.T_bytea:
-		return math.MaxInt64, true
-	case oid.T_varchar, oid.T_bpchar:
-		return int64(fd.Mod - headerSize), true
-	default:
-		return 0, false
-	}
-}
-
-func (fd fieldDesc) PrecisionScale() (precision, scale int64, ok bool) {
-	switch fd.OID {
-	case oid.T_numeric, oid.T__numeric:
-		mod := fd.Mod - headerSize
-		precision = int64((mod >> 16) & 0xffff)
-		scale = int64(mod & 0xffff)
-		return precision, scale, true
-	default:
-		return 0, 0, false
-	}
-}
-
-// ColumnTypeScanType returns the value type that can be used to scan types into.
-func (rs *rows) ColumnTypeScanType(index int) reflect.Type {
-	return rs.colTyps[index].Type()
-}
-
-// ColumnTypeDatabaseTypeName return the database system type name.
-func (rs *rows) ColumnTypeDatabaseTypeName(index int) string {
-	return rs.colTyps[index].Name()
-}
-
-// ColumnTypeLength returns the length of the column type if the column is a
-// variable length type. If the column is not a variable length type ok
-// should return false.
-func (rs *rows) ColumnTypeLength(index int) (length int64, ok bool) {
-	return rs.colTyps[index].Length()
-}
-
-// ColumnTypePrecisionScale should return the precision and scale for decimal
-// types. If not applicable, ok should be false.
-func (rs *rows) ColumnTypePrecisionScale(index int) (precision, scale int64, ok bool) {
-	return rs.colTyps[index].PrecisionScale()
-}