localstore: fix data races on subscriptions

[acud]

This PR fixes data races that were detected by the go race detector:

WARNING: DATA RACE
Write at 0x00c0082deb80 by goroutine 2475:
  runtime.slicecopy()
      /usr/local/go/src/runtime/slice.go:197 +0x0
  github.com/ethersphere/swarm/storage/localstore.(*DB).SubscribePull.func2()
      /home/growlie/go/src/github.com/ethersphere/swarm/storage/localstore/subscription_pull.go:173 +0x38b
  github.com/ethersphere/swarm/network/stream.(*Registry).serverCollectBatch()
      /home/growlie/go/src/github.com/ethersphere/swarm/network/stream/stream.go:962 +0xb22
  github.com/ethersphere/swarm/network/stream.(*Registry).serverHandleGetRange()
      /home/growlie/go/src/github.com/ethersphere/swarm/network/stream/stream.go:443 +0x682
  github.com/ethersphere/swarm/network/stream.(*Registry).HandleMsg.func1.1()
      /home/growlie/go/src/github.com/ethersphere/swarm/network/stream/stream.go:188 +0xa6b

Previous read at 0x00c0082deb80 by goroutine 2477:
  github.com/ethersphere/swarm/storage/localstore.(*DB).triggerPullSubscriptions()
      /home/growlie/go/src/github.com/ethersphere/swarm/storage/localstore/subscription_pull.go:210 +0x10d
  github.com/ethersphere/swarm/storage/localstore.(*DB).put()
      /home/growlie/go/src/github.com/ethersphere/swarm/storage/localstore/mode_put.go:150 +0x5aa
  github.com/ethersphere/swarm/storage/localstore.(*DB).Put()
      /home/growlie/go/src/github.com/ethersphere/swarm/storage/localstore/mode_put.go:41 +0x234
  github.com/ethersphere/swarm/storage.(*NetStore).Put()
      /home/growlie/go/src/github.com/ethersphere/swarm/storage/netstore.go:116 +0x479
  github.com/ethersphere/swarm/storage.(*LNetStore).Put()
      <autogenerated>:1 +0xc9
  github.com/ethersphere/swarm/storage.(*FileStore).Put()

[janos]

Thanks for finding and fixing this.