examples: adding more examples
linkpack: fixing bug about mempool and thread

In linkpack class, the thread can start before mempool construction
(assignation to nullptr). So we move the mempool initialization into the
constructor and move the thread initialisation after the mempool
initialisation.
log: adding a logger class

The bench::log class is used in many place to write formatted string
into a file descriptor.

- Updating main.cpp to use logger and assign default log level.
- Updating models in models.hpp to use logger.