Data Migrator: Improvements and tests

Summary:
Process items in-memory
EWS: Migrate distlist description, clarify AdditionProperties use
EWS: cleanup
EWS: Speed up listing folders by usign FolderClass filter
EWS: Use ItemSet to minimize number of jobs
Optimize data structures
More tests and fixes

Test Plan: ./phpunit

Reviewers: #hkccp_developers

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4857