improve initialize() performance

- use async.each instead of eachSeries

This improves performance in proportion to the number of concurrent operations the datastore is capable of. For example, on a 4-core machine with HT, performance improvement is roughly 6x. Time to create 304 tables (approx 11k columns total) decreased from 2 minutes to ~17 seconds with `sails-postgres` adapter. Performance seems unchanged in `sails-disk` and `sails-memory`.