added last activity timestamp for vhosts

[kickster97]

Related to this card: https://trello.com/c/r7pGcwWb Logs a last activity timestamp for each Vhost (If a vhost has connections, it is active)

[kickster97]

Needs to store the timestamp persistently, now it only lives when LavinMQ runs.

[dentarg]

This will be a feature that RabbitMQ doesn't have, right? Are we keeping a list of differences somewhere? I think it would be good to do.

Also, don't forget to update the API schema with this property. You find it at https://github.com/cloudamqp/lavinmq/blob/f43c8632b2807442a55b95cb826f6dfae25b3a98/openapi/schemas/vhosts.yaml#L6-L11

[github-actions[bot]]

Main benchmark 'Average publish rate: 63941.9 msgs/s' 'Average consume rate: 64759.9 msgs/s'

PR benchmark 'Average publish rate: 66803.8 msgs/s' 'Average consume rate: 65297.7 msgs/s'

Keep in mind, these numbers are not representative of LavinMQ's peak performance. It is rather an indication of how the changes of this pull request affects the performance of the main branch.