store: Make postponed index creation robust against concurrent builds

Acquire a fresh connection for each postponed index instead of holding
one for the whole run, which can take hours. Before starting a
`CREATE INDEX CONCURRENTLY`, wait for any index creation already running
in the deployment's schema to finish, using `index_creation_is_running`
to poll `pg_stat_progress_create_index`.

Have CreateIndex::name return a string slice to avoid an allocation.