add close flag into wsConnection to avoid duplicate calls of CloseFunc

[vlad-tokarev]

Fixes: #2679

I attempted the simplest way to resolve this issue by adding a guard flag. Unfortunately, the control flow in the run function is quite confusing. The next steps should likely focus on making it more predictable and straightforward.

To illustrate what I mean about the confusing control flow, consider the following example:

1. Let's assume that the context in the run function gets canceled due to a deadline set in the InitFunc. This was one of the workarounds proposed to implement the closing transport for expiring tokens.
2. In such a scenario, closeOnCancel would be triggered, subsequently closing the wsConnection.
3. After that, the normal point of exit from the subroutine would be the error handler in NextMessage(), which isn't straightforward either.
4. Due to this exit, a deferred function would be called, trying to close the wsConnection again. This happens with the CloseAbnormalClosure code, even though the closure might be normal.
5. A second call to close() doesn't panic or return an error since all errors are ignored inside wsConnection.Close(), but CloseFunc would be invoked again.

I also thought about removing the deferred close code and adding explicit closures at every exit point of the subroutine. However, this approach seems riskier than adding a guard flag. There's no guarantee that resources would be released, for instance, if a panic occurred.

I would appreciate any feedback. Thank you.

I have:

• Added tests covering the bug/feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 75.691% (-0.08%) from 75.767% when pulling 30599b5 on vlad-tokarev:fix/close-twice into af4d394 on 99designs:master.

[vlad-tokarev]

Fixed linter issues

[StevenACoffman]

Thanks for the PR! Looking forward to your next!