Gateway improvements for Plan Import

[dandelany]

Additional improvements to Plan Import feature in aerie-gateway which were deferred from #1154 and #1401 :

• Activity directives should be inserted using a bulk insert rather than one query per activity - see this PR comment for details: Feature: Plan import plandev-gateway#97 (comment)
• Imports should be done as an atomic transaction, so the entire import either succeeds or fails and can't be left in an inconsistent state: Feature: Plan import plandev-gateway#97 (comment)