fix(proposals): Incremental proposal key for zero proposals (#8005)

Change the proposal's unique key to an atomic counter instead of using a randomly generated key.

(cherry picked from commit a515d0d)
(cherry picked from commit 2aa3d3e)

worker/proposal.go

@@ -24,15 +24,15 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/pkg/errors"
-	ostats "go.opencensus.io/stats"
-	tag "go.opencensus.io/tag"
-	otrace "go.opencensus.io/trace"
-
 	"github.com/dgraph-io/dgraph/conn"
 	"github.com/dgraph-io/dgraph/protos/pb"
 	"github.com/dgraph-io/dgraph/schema"
 	"github.com/dgraph-io/dgraph/x"
+
+	"github.com/pkg/errors"
+	ostats "go.opencensus.io/stats"
+	tag "go.opencensus.io/tag"
+	otrace "go.opencensus.io/trace"
 )
 
 const baseTimeout time.Duration = 4 * time.Second