Minimize number of User/UpdateJob's

Summary:
Use two Laravel features:

• Dispatch all jobs at the end of transaction (after commit)
• Make User/UpdateJob unique in the queue, i.e. there will be no new jobs added to the queue for specific user if there's another job in there already (unless it's older than 60 seconds). It will limit how often we connect to IMAP/LDAP.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4610