Make sure schema changes are applied sequentially
Make sure schema changes are applied sequentially

Cassandra does not support concurrent schema changes. For this reason,
we have been very careful to apply all schema changes sequentially:

- Start one RB instance at a time in a deploy.
- Start one worker at a time in service-runner.
- Create one cf/keyspace at a time in the storage backend, and wait for
schema agreement.

However, multi_content_bucket introduced a regression by using .map() to
apply several schema changes in parallel. This did not work very well
with Cassandra.

This patch replaces .map() with .each(), returning to sequential
execution.

d00rman/restbase,wikimedia/mediawiki-services-restbase,wikimedia/mediawiki-services-restbase,milimetric/restbase,wikimedia/restbase,cscott/restbase,milimetric/restbase,d00rman/restbase,wikimedia/restbase,cscott/restbase,Pchelolo/restbase,Pchelolo/restbase