Protect map and queue variable access with mutex, add test

[C-Pro]

This PR should fix a couple of possible races by adding mutex lock where variable access was not protected.

Background:

I've got a race in centrifuge-go while running my application tests:

WARNING: DATA RACE
Write at 0x00c000488630 by goroutine 9:
  runtime.mapassign_faststr()
      /usr/local/go/src/runtime/map_faststr.go:203 +0x0
  github.com/centrifugal/centrifuge-go.(*Client).NewSubscription()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:191 +0x6bd
  github.com/pintu-crypto/exchange/backend/account/view/fake.TestFakeCentrifuge()
      /runner/_work/exchange/exchange/backend/account/view/fake/fake_test.go:176 +0x135e
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      /usr/local/go/src/testing/testing.go:1629 +0x47

Previous read at 0x00c000488630 by goroutine 122:
  runtime.mapaccess2_faststr()
      /usr/local/go/src/runtime/map_faststr.go:108 +0x0
  github.com/centrifugal/centrifuge-go.(*Client).handlePush()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:699 +0x265
  github.com/centrifugal/centrifuge-go.(*Client).handle()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:667 +0x467
  github.com/centrifugal/centrifuge-go.(*Client).readOnce()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:605 +0x12a
  github.com/centrifugal/centrifuge-go.(*Client).reader()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:612 +0xad
  github.com/centrifugal/centrifuge-go.(*Client).startReconnecting.func5()
      /home/runner/go/pkg/mod/github.com/centrifugal/[email protected]/client.go:950 +0x64

I believe the reason was an unprotected c.subs map read in handlePush. I added the test to reproduce and another race popped out in runHandlerSync, looks like c.cbQueue also can be mutated concurrently, so I added mutex there too.

Please take a look.

[FZambia]

Uff, sorry for the delay, I've seen this PR but totally forgot about it. Many thanks @C-Pro