Currently, the backlog used for MV flow control is only updated
after we generate view updates as a result of a write request.
However, when the resources are no longer used, we should also
notice that to prevent excessive slowdowns caused by the MV
flow control calulating the delays based of an outdated, large
backlog.
This patch makes it so the backlogs are updated every time
a view update finishes, and not only when the updates start.
Fixes#18783Closesscylladb/scylladb#18804
2cafa573df