Use Xid instead of UUID for SubscriptionID (yorkie-team#142)

Xid(12bytes) smaller than UUID and it's sortable. It is useful if the
SubscriptionID can be sortable.

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/grpc-ecosystem/go-grpc-middleware v1.0.0
 	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.0
 	github.com/prometheus/client_golang v0.9.3
+	github.com/rs/xid v1.2.1
 	github.com/spf13/cobra v1.1.1
 	github.com/stretchr/testify v1.6.2-0.20201103103935-92707c0b2d50
 	github.com/tidwall/pretty v1.0.0 // indirect

go.sum

@@ -322,6 +322,8 @@ github.com/quasilyte/regex/syntax v0.0.0-20200407221936-30656e2c4a95/go.mod h1:r
 github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af/go.mod h1:XWv6SoW27p1b0cqNHllgS5HIMJraePCO15w5zCzIWYg=
 github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=
 github.com/rogpeppe/go-internal v1.6.0/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
+github.com/rs/xid v1.2.1 h1:mhH9Nq+C1fY2l1XIpgxIiUOfNpRBYH1kKcr+qfKgjRc=
+github.com/rs/xid v1.2.1/go.mod h1:+uKXf+4Djp6Md1KODXJxgGQPKngRmWyn10oCKFzNHOQ=
 github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
 github.com/ryancurrah/gomodguard v1.1.0 h1:DWbye9KyMgytn8uYpuHkwf0RHqAYO6Ay/D0TbCpPtVU=
 github.com/ryancurrah/gomodguard v1.1.0/go.mod h1:4O8tr7hBODaGE6VIhfJDHcwzh5GUccKSJBU0UMXJFVM=

yorkie/backend/pubsub/pubsub.go

@@ -19,7 +19,7 @@ package pubsub
 import (
 	"errors"
 
-	"github.com/google/uuid"
+	"github.com/rs/xid"
 
 	"github.com/yorkie-team/yorkie/pkg/document/time"
 	"github.com/yorkie-team/yorkie/pkg/types"
@@ -49,7 +49,7 @@ type Subscription struct {
 // NewSubscription creates a new instance of Subscription.
 func NewSubscription(subscriber types.Client) *Subscription {
 	return &Subscription{
-		id:         uuid.New().String(),
+		id:         xid.New().String(),
 		subscriber: subscriber,
 		// [Workaround] The channel buffer size below avoids stopping during
 		//   event issuing to the events channel. This bug occurs in the order