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.

Resources:

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

[mseri]

Are also xcp-networkd and wsproxy affected?

[gaborigloi]

wsprox is: xapi-project/wsproxy#13 , but xcp-networkd isn't

[edwintorok]

FWIW here is an example of using the Logs module: https://github.com/xapi-project/testarossa/blob/master/lib/context.ml. In this case I used the blocking version of logs, because it was all CLI code and I actually wanted the blocking behaviour.
For the daemons we want to use the Lwt reporter probably: http://erratique.ch/software/logs/doc/Logs_lwt.html#report_ex