Enable reopening log files

[bhjortsberg]

Once a Logger is configured the application keeps the file open. This is due to that LogStreamReferenceMap is keeping a reference to the FileStream object and unregistering the Logger will never close the file.

Switching to weak_ptr in LogStreamReferenceMap i.e. not holding a reference makes unregistering logger close the file if no other reference to FileStream object exist.

The use case is for logrotation where logrotation removes the logfile and sends a SIGHUP to the application. This fix makes it possible to reconfigure the logger to reopen the log file. I added a sample to illustrate the use case.

This is a

• [ ] Breaking change
• [ ] New feature
• [x] Bugfix

I have

• [x] Merged in the latest upstream changes
• [ ] Updated CHANGELOG.md
• [ ] Updated README.md
• [x] Run the tests

[bhjortsberg]

Apparently unit test fails, I haven't looked at them yet. Any comment on this change, does it break the architecture? Or is it just the tests that need to be updated?

[bhjortsberg]

Tests passes when I run them locally with my latest update.

Travis CI build seem to have some problem with cmake when building googletest.

[BrainStone]

I would love to see this added, as without using the linux logrotate utility is impossible, as that relies on the program being able to close and reopen the log file.