Easier tailing of log files

This creates a `/var/log/<component>/<component>_current.txt` symlink pointing at the current log file (and updates it on every log rotation). This gets the key benefit of https://github.com/Metaswitch/clearwater-infrastructure/issues/63, without all the upheaval of actually moving to logrotate.

Tested by running `tail -F /var/log/sprout/sprout_current.txt` and seeing continued output across the hour boundary.
    /home/ubuntu/src/ralf/modules/cpp-common/src/logger.cpp: In member function ‘void Logger::cycle_log_file(const Logger::timestamp_t&)’:
    /home/ubuntu/src/ralf/modules/cpp-common/src/logger.cpp:231:25: warning: ignoring return value of ‘int symlink(const char*, const char*)’, declared with attribute warn_unused_result [-Wunused-result]

Tut tut.