The code would previously `announce` schema mutations once per each keyspace and
once per each table. This can be reduced to two calls of `announce`:
once to create all keyspaces, and once to create all tables.
This should be further reduced to a single `announce` in the future.
Left a FIXME.
Motivation: after migrating to Raft, each `announce` will require a
`read_barrier` to achieve linearizability of schema operations. This
introduces latency, as it requires contacting a leader which then must
contact a quorum. The fewer announce calls, the better. Also, if all
sub-operations are reduced to a single `announce`, we get atomicity -
either all of these sub-operations succeed or none do.
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