fix(dgraph): fixing race condition in disk storage sync and save

[aman-bansal]

This PR fix following race condition

Write at 0x00c000306e00 by goroutine 242:
  github.com/dgraph-io/ristretto/z.(*MmapFile).Truncate()
      /home/agent/go/pkg/mod/github.com/dgraph-io/[email protected]/z/file_linux.go:35 +0x479
  github.com/dgraph-io/ristretto/z.(*MmapFile).AllocateSlice()
      /home/agent/go/pkg/mod/github.com/dgraph-io/[email protected]/z/file.go:148 +0x279
  github.com/dgraph-io/dgraph/raftwal.(*wal).AddEntries()
      /home/agent/work/4c273890c7f2d4e2/raftwal/wal.go:184 +0x8fa
  github.com/dgraph-io/dgraph/raftwal.(*DiskStorage).Save()
      /home/agent/work/4c273890c7f2d4e2/raftwal/storage.go:332 +0xf8
  github.com/dgraph-io/dgraph/conn.(*Node).SaveToStorage()
      /home/agent/work/4c273890c7f2d4e2/conn/node.go:288 +0xf7
  github.com/dgraph-io/dgraph/worker.(*node).Run()
      /home/agent/work/4c273890c7f2d4e2/worker/draft.go:1195 +0x40c

Previous read at 0x00c000306e00 by goroutine 248:
  github.com/dgraph-io/ristretto/z.(*MmapFile).Sync()
      /home/agent/go/pkg/mod/github.com/dgraph-io/[email protected]/z/file.go:161 +0x1a4
  github.com/dgraph-io/dgraph/raftwal.(*DiskStorage).Sync()
      /home/agent/work/4c273890c7f2d4e2/raftwal/storage.go:380 +0x96
  github.com/dgraph-io/dgraph/x.StoreSync()
      /home/agent/work/4c273890c7f2d4e2/x/x.go:1065 +0x1e2

Goroutine 242 (running) created at:
  github.com/dgraph-io/dgraph/worker.(*node).InitAndStartNode()
      /home/agent/work/4c273890c7f2d4e2/worker/draft.go:1737 +0x849
  github.com/dgraph-io/dgraph/worker.StartRaftNodes()
      /home/agent/work/4c273890c7f2d4e2/worker/groups.go:147 +0xbd9
  github.com/dgraph-io/dgraph/dgraph/cmd/alpha.run.func4()
      /home/agent/work/4c273890c7f2d4e2/dgraph/cmd/alpha/run.go:760 +0x73

Goroutine 248 (running) created at:
  github.com/dgraph-io/dgraph/worker.(*node).Run()
      /home/agent/work/4c273890c7f2d4e2/worker/draft.go:1076 +0x31ef
==================

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This change is