Move from Lwt_log to the logs library

[gaborigloi]

We currently use the Lwt_log library for logging in this repo. As ocsigen/lwt#468 (comment) said, Lwt_log might be deprecated in the future.
Therefore, we should move this repo to use the logs libray instead, which is the replacement recommended in the above thread. Its Logs_lwt library provides logging functions for Lwt.

We have to move the nbd library to logs first: xapi-project/nbd#113

Resources:

• Manual for the Lwt_log module in Lwt
• GitHub repo of the logs library