db/hints/resource_manager: Correctly account resources in space_watchdog
db/hints/resource_manager: Correctly account resources in space_watchdog

A db::hints::resource_manager manages the resources for one or two
db::hints::managers. Each of these can be using the same or different
devices. The db::hints::space_watchdog periodically checks whether
each manager is within their resource allocation, and if not disables
it.

The watchdog iterates over the managers and accounts for the total
size they are using. This is wrong, since it can account in the same
variable the size consumed by managers using different devices.

We fix this while taking advantage of the fact that on_timer is now
called in the context of a seastar::thread, instead of using future
combinators.

Fixes #3821

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla