This is another part of splitting Scylla initialization into two phases:
local and remote parts. Performing queries is done with `storage_proxy`,
so for local queries we want to initialize it before we initialize
services specific to cluster communication such as `gossiper`,
`messaging_service`, `storage_service`.
`system_keyspace` should also be initialized after `storage_proxy` (and
is after this patch) so in the future we'll be able to merge the
multiple initialization steps of `system_keyspace` into one (it only
needs the local part to work).
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