allow to list all backups on every backups through the 'magic' -1 value + remove some gevent/parallelization code which is hard to maintain

- Transparently log user details
- Add log origin

The size of the data to send/receive was evaluated before converting the
data resulting in corruptions while fetching data from remote.

Avoid provisioning some "web-only" components such as cache, db, etc.
Modules might be lazy-loaded by subcommands at will.

This new backend interact with the bui-monitor introduced in fff83dd5
A few things were refactored to take advantage of the queries
parallelization.

Handle a pool of burp client processes to have a more predictable amount
of burp client and allow some process parallelisation.