* Move callback function in thread_pool ctor

* Added const qualifiers to logger.h

* Remove unused includes from file_helper-inl.h

* Fix comments and remove unused include from helpers-inl.h

* Fix typo in comment for set_default_logger method.

* Use `std::move` for `old_logger` in `set_default_logger`.

* Use std::move in example

* Wrap `main` content in try block for exception safety.

* Added coverity to ci