bmt: fix data races in Hasher

[janos]

This PR fixes two data races found in bmt hasher.

Fixes are minimal, but maybe locking can be improved a bit more.

==================
WARNING: DATA RACE
Write at 0x00c000080418 by goroutine 87:
  github.com/ethersphere/swarm/bmt.(*Hasher).WriteSection()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:445 +0x71

Previous read at 0x00c000080418 by goroutine 39:
  github.com/ethersphere/swarm/bmt.(*Hasher).Sum()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:338 +0xae
  github.com/ethersphere/swarm/bmt.syncHash()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:454 +0xd9
  github.com/ethersphere/swarm/bmt.testHasherCorrectness()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:318 +0x32b
  github.com/ethersphere/swarm/bmt.TestSyncHasherCorrectness.func1()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:163 +0x2ed
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:909 +0x199

Goroutine 87 (running) created at:
  github.com/ethersphere/swarm/bmt.(*Hasher).Write()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:398 +0x3f9
  github.com/ethersphere/swarm/bmt.syncHash()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:453 +0xba
  github.com/ethersphere/swarm/bmt.testHasherCorrectness()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:318 +0x32b
  github.com/ethersphere/swarm/bmt.TestSyncHasherCorrectness.func1()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:163 +0x2ed
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:909 +0x199

Goroutine 39 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:960 +0x651
  github.com/ethersphere/swarm/bmt.TestSyncHasherCorrectness()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:154 +0x163
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:909 +0x199
==================

==================
WARNING: DATA RACE
Read at 0x00c0000868d8 by goroutine 170:
  github.com/ethersphere/swarm/bmt.(*Hasher).WriteSection()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:445 +0x55

Previous write at 0x00c0000868d8 by goroutine 166:
  github.com/ethersphere/swarm/bmt.(*Hasher).Write()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:367 +0x9e
  github.com/ethersphere/swarm/bmt.TestBMTWriterBuffers.func1.1()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:262 +0x23a
  github.com/ethersphere/swarm/bmt.TestBMTWriterBuffers.func1.2()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:280 +0x34

Goroutine 170 (running) created at:
  github.com/ethersphere/swarm/bmt.(*Hasher).Write()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt.go:398 +0x3f9
  github.com/ethersphere/swarm/bmt.TestBMTWriterBuffers.func1.1()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:262 +0x23a
  github.com/ethersphere/swarm/bmt.TestBMTWriterBuffers.func1.2()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:280 +0x34

Goroutine 166 (running) created at:
  github.com/ethersphere/swarm/bmt.TestBMTWriterBuffers.func1()
      /Users/janos/go/src/github.com/ethersphere/swarm/bmt/bmt_test.go:279 +0x5ca
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:909 +0x199
==================

[nolash]

what effect does it have on performance?

[janos]

I do not know, but it has effect on correctness.

[nolash]

Thanks for finding this. Let's revisit the optimizations later. Anyway this doesn't affect performance much.