Take `query_processor` and `database` references directly, not through
`sharded<...>&`. This is now possible because we moved `query_processor`
and `database` construction early, so by the time `system_keyspace` is
started, the services it depends on were also already started.
Calls to `_qp.local()` and `_db.local()` inside `system_keyspace` member
functions can now be replaced with direct uses of `_qp` and `_db`.
Runtime assertions for dependant services being initialized are gone.
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