Reindex timeout fixes

These changes appear to make the reindexing work without timing out. I think the main issue was that all four models we're being reindexed at once, so they were all treading on each others toes. By using `mapSeries` they run one model at a time, which seems to reduce load (though memory usage is still quite high during the reindex?).

```
chrism@ajax:/data/vhost/popit.staging.mysociety.org/popit$ time bin/replace-database  ~mark/popolo-test-mongo- popit_staging_mark-test
...
Reindexing the database: popit_staging_mark-test
Sending elasticsearch bulk command for Person
Completed elasticsearch bulk command for Person
Sending elasticsearch bulk command for Organization
Completed elasticsearch bulk command for Organization
Sending elasticsearch bulk command for Membership
Completed elasticsearch bulk command for Membership
Re-indexed 28657 docs from popit_staging_mark-test

real    1m56.804s
user    1m7.888s
sys     0m5.556s
chrism@ajax:/data/vhost/popit.staging.mysociety.org/popit$
```