feat(logger): use single instance of bunyan logger (fix rolling files)

[camfou]

Hello !
We need to retrieve the bunyan instance in our hooks and protocols.
But, if we create another bunyan logger with the same config with rolling file option we may run into issues (trying to close the logfile during another logger writing)
This PR transforms the logger in singleton.

Regards,
Camille

[coveralls]

Coverage remained the same at 79.75% when pulling 5afda8e on camfou:single_logger_instance into 46b118c on anvilresearch:master.

[camfou]

Bad idea, the returned instance is not a bunyan logger but an express-bunyan-logger which is a middleware instance...