Mojo::Log is a simple logger class. It holds a filehandle once it writes to
a log, and changing "path" in Mojo::Log does not open a new filehandle for
logging.

Mojo::Log::Role::Clearable is a role that provides a "clear_handle" method
and automatically calls it when "path" in Mojo::Log is modified, so the
logging handle is reopened at the new path.

The "clear_handle" method can also be used to reopen the logging handle
after log rotation.