Fix memory leak in the p2p channel

[soffokl]

No description provided.

[Snawoot]

note data race and panic:

==================
WARNING: DATA RACE
Write at 0x00c000408028 by goroutine 30:
  github.com/mysteriumnetwork/node/p2p.(*channel).Close.func1()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel.go:493 +0x829
  sync.(*Once).doSlow()
      /usr/local/go/src/sync/once.go:68 +0x127
  sync.(*Once).Do()
      /usr/local/go/src/sync/once.go:59 +0x46
  github.com/mysteriumnetwork/node/p2p.(*channel).Close()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel.go:462 +0x111
  github.com/mysteriumnetwork/node/p2p.TestChannel_Send_To_When_Peer_Starts_Later()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:224 +0x188
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1259 +0x22f
  testing.(*T).Run·dwrap·21()
      /usr/local/go/src/testing/testing.go:1306 +0x47
Previous read at 0x00c000408028 by goroutine 35:
  github.com/mysteriumnetwork/node/p2p.(*channel).localSendLoop()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel.go:368 +0x5e
  github.com/mysteriumnetwork/node/p2p.(*channel).launchReadSendLoops·dwrap·6()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel.go:251 +0x39
Goroutine 30 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:1306 +0x726
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:1598 +0x99
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1259 +0x22f
  testing.runTests()
      /usr/local/go/src/testing/testing.go:1596 +0x7ca
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:1504 +0x9d1
  main.main()
      _testmain.go:129 +0x324
Goroutine 35 (finished) created at:
  github.com/mysteriumnetwork/node/p2p.(*channel).launchReadSendLoops()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel.go:251 +0x1e4
  github.com/mysteriumnetwork/node/p2p.createTestChannels()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:338 +0x4a4
  github.com/mysteriumnetwork/node/p2p.TestChannel_Send_To_When_Peer_Starts_Later()
      /home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:214 +0x44
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1259 +0x22f
  testing.(*T).Run·dwrap·21()
      /usr/local/go/src/testing/testing.go:1306 +0x47
==================
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc56482]
goroutine 86 [running]:
github.com/mysteriumnetwork/node/p2p.reopenChannel(0xc000408000)
	/home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:282 +0x62
github.com/mysteriumnetwork/node/p2p.TestChannel_Send_To_When_Peer_Starts_Later.func1()
	/home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:228 +0x65
created by github.com/mysteriumnetwork/node/p2p.TestChannel_Send_To_When_Peer_Starts_Later
	/home/gitlab-runner/go/src/github.com/mysteriumnetwork/node/p2p/channel_test.go:226 +0x279```

[codecov-commenter]

Codecov Report

Merging #4801 (f1e7015) into master (8bfa049) will decrease coverage by 0.02%.
The diff coverage is 80.76%.

❗ Current head f1e7015 differs from pull request most recent head eefc69c. Consider uploading reports for the commit eefc69c to get more accurate results

@@            Coverage Diff             @@
##           master    #4801      +/-   ##
==========================================
- Coverage   39.26%   39.24%   -0.03%     
==========================================
  Files         356      356              
  Lines       19136    19138       +2     
==========================================
- Hits         7514     7510       -4     
- Misses      10899    10903       +4     
- Partials      723      725       +2     

Impacted Files	Coverage Δ
nat/traversal/pinger.go	77.51% <ø> (-0.39%)	⬇️
p2p/channel.go	66.06% <80.76%> (-2.32%)	⬇️
session/pingpong/hermes_promise_handler.go	59.29% <0.00%> (+0.77%)	⬆️

---

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