Our goal is to switch system.local table to schema
commitlog and stop doing flushes when we write to it.
This means it would be incorrect to read from this
table until schema commitlog is replayed.
On the other hand, we need truncation records
to be loaded before we start replaying schema
commitlog, since commitlog_replayer relies on them.
In this commit we inline the system_keyspace::setup
function and split its content into two parts. In
the first part, before schema commitlog replay,
we load truncation records. It's safe to load
them before schema commitlog replay since we intend
to let the flushes on writes to system.truncated
table. In the second part, after schema commitlog replay,
we do the rest of the job - build_bootstrap_info and
db::schema_tables::save_system_schema.
We decided to inline this function since there is
very low cohesion between the actions it's performing.
It's just simpler to reason about them individually.
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