Update TODO list

* Reflect Pierrick's current activity
* Add benchmarking idea
Rework core::file::readline implementation

* Avoid useless redundancy between core::file::readline() and
  core::file::readline(size_t)
* Fix a bug due to using size_t instead of ssize_t
* Fix performance issue by using a thread-safe static storage
* Fix string interface usage
Fix performance and conformance issue in string to long conversion

* std::stringstream proved to be very slow (additional copy?), using
  strtol instead is *really** faster
* raise an exception if conversion failed
* test the whole stuff