Data race in handleIntroductions

[calmh]

I think this one is new, v0.14.11+1-g3088dac, maybe same root cause as #3735

==================
WARNING: DATA RACE
Read at 0x00c4200a6510 by goroutine 61:
  runtime.mapaccess1()
      /usr/local/go/src/runtime/hashmap.go:272 +0x0
  github.com/syncthing/syncthing/lib/model.(*Model).handleIntroductions()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:923 +0x3ea
  github.com/syncthing/syncthing/lib/model.(*Model).ClusterConfig()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:876 +0x1ec8
  github.com/syncthing/syncthing/lib/model.TestIntroducer()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model_test.go:553 +0x67f
07:48:13 WARNING: Stopping folder "folder1" - folder path empty
07:48:13 INFO: Skipping folder folder1 scan due to folder error: folder path empty
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9

Previous write at 0x00c4200a6510 by goroutine 71:
  runtime.mapassign1()
      /usr/local/go/src/runtime/hashmap.go:442 +0x0
  github.com/syncthing/syncthing/lib/model.(*Model).addFolderLocked()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:308 +0x4cc
  github.com/syncthing/syncthing/lib/model.(*Model).RestartFolder()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:365 +0x142
  github.com/syncthing/syncthing/lib/model.(*Model).CommitConfiguration()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:2373 +0x8fe
  github.com/syncthing/syncthing/lib/config.(*Wrapper).notifyListener()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/config/wrapper.go:170 +0x23a

Goroutine 61 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:646 +0x52f
  testing.RunTests.func1()
      /usr/local/go/src/testing/testing.go:793 +0xb9
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9
  testing.RunTests()
      /usr/local/go/src/testing/testing.go:799 +0x4ba
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:743 +0x12f
  main.main()
      github.com/syncthing/syncthing/lib/model/_test/_testmain.go:154 +0x1b8

Goroutine 71 (finished) created at:
  github.com/syncthing/syncthing/lib/config.(*Wrapper).notifyListeners()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/config/wrapper.go:164 +0x238
  github.com/syncthing/syncthing/lib/config.(*Wrapper).replaceLocked()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/config/wrapper.go:157 +0x5ad
  github.com/syncthing/syncthing/lib/config.(*Wrapper).SetDevice()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/config/wrapper.go:209 +0x27b
  github.com/syncthing/syncthing/lib/model.(*Model).AddConnection()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model.go:1367 +0xc04
  github.com/syncthing/syncthing/lib/model.TestIntroducer.func2()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model_test.go:514 +0x52d
  github.com/syncthing/syncthing/lib/model.TestIntroducer()
      /Users/jb/jenkins/workspace/syncthing-mac/src/github.com/syncthing/syncthing/lib/model/model_test.go:539 +0x4c8
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9
==================