Send advisory lock requests to the master

[georgeclaghorn]

Fixes running migrations in Rails 5.2, which takes an advisory lock on a replica connection and attempts to release it on the primary connection:

[replica] (0.1ms)  SELECT GET_LOCK('5059312551837065865', 0)
[replica] (0.4ms)  SELECT `schema_migrations`.`version` FROM `schema_migrations` ORDER BY `schema_migrations`.`version` ASC
Migrating to IndexRemindersOnDeliveredAndRemindAt (20180301142256)
[primary] (29.2ms)  CREATE  INDEX `index_reminders_on_delivered_and_remind_at`  ON `reminders` (`delivered`, `remind_at`) 
[primary] (0.3ms)  BEGIN
[primary] ActiveRecord::SchemaMigration Create (0.6ms)  INSERT INTO `schema_migrations` (`version`) VALUES ('20180301142256')
[primary] (0.5ms)  COMMIT
[primary] ActiveRecord::InternalMetadata Load (0.3ms)  SELECT  `ar_internal_metadata`.* FROM `ar_internal_metadata` WHERE `ar_internal_metadata`.`key` = 'environment' LIMIT 1
[primary] (0.2ms)  BEGIN
[primary] (0.2ms)  COMMIT
[primary] (0.3ms)  SELECT RELEASE_LOCK('5059312551837065865')

Failed to release advisory lock

[bleonard]

Thanks @georgeclaghorn !