CYB-352 Return DB connections to the pool after done with using raw SQL

 * use a block yielding to the execution statement so that the connection is only used
   within said block
 * decrease sidekiq concurrency, so fewer connections are used at once.

[CYB-352 Return DB connections to the pool after done with using raw SQL](https://jira.ets.berkeley.edu/jira/browse/CYB-352)

{ Can Merge once the data populating script is stopped }