Improve performance of autosyncable_entries query

[rywall]

This query is taking 8s on staging which causes the autosync task to operate extremely slowly.

https://github.com/culturecode/stagehand/blob/8626e3a1e22544d929267cfa347eee41c71faa72/lib/stagehand/staging/synchronizer.rb#L93

Stagehand::Staging::CommitEntry Load (8161.6ms)  SELECT  `stagehand_commit_entries`.* FROM `stagehand_commit_entries` LEFT OUTER JOIN (SELECT session, MAX(id) AS commit_id FROM `stagehand_commit_entries` WHERE `stagehand_commit_entries`.`commit_id` IS NULL AND `stagehand_commit_entries`.`operation` = 'commit_start' GROUP BY `stagehand_commit_entries`.`session`) AS active_commits ON active_commits.session = stagehand_commit_entries.session AND active_commits.commit_id <= stagehand_commit_entries.id LEFT OUTER JOIN (SELECT DISTINCT record_id, table_name FROM `stagehand_commit_entries` WHERE (`stagehand_commit_entries`.`commit_id` IS NOT NULL)) AS contained ON contained.record_id = stagehand_commit_entries.record_id AND contained.table_name = stagehand_commit_entries.table_name WHERE `stagehand_commit_entries`.`operation` IN ('insert', 'update', 'delete') AND (active_commits.commit_id IS NULL) AND (id <= 115791519) AND `stagehand_commit_entries`.`commit_id` IS NULL AND (contained.record_id IS NULL) ORDER BY FIELD(operation, 'delete','update','insert'), id DESC LIMIT 1000

[njakobsen]

This will be resolved by #77