system_keyspace: cache local host id before flushing
system_keyspace: cache local host id before flushing

Later in this series the ME sstable format is made default, which
means that `system.local` will likely be written as ME.

Since, in ME, originating host id is a part of sstable stats metadata,
the local host id needs to either already be cached by the time
`system.local` is flushed, or to somehow be special-case-ignored when
flushing `system.local`.

The former (done here) is optimistic (cache before flush), but the
alternative would be an abstraction violation and would also cost a
little time upon each sstable write.

(Cache-before-flush could be undone by catching any exceptions during
flush and un-caching, but inability to `co_await` in catch clauses
makes the code look rather awkward.  And there is no need to bother
because bootstrap failures should be fatal anyway)

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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