Improve external database enrolment sync performance by running sync in parallel

On a big instances (e.g. a university in the begging of a semester) a huge number of enrolment activities can run for a long time. 

For example recently we've seen following:

• new enrolment - 144 359 events
• re-enrolling - 1 175 events
• assigning new roles - 2 233 events
• un-assigning roles - 1 079 events

This took more that 12 hours.

We propose rather than run all those events in one long running task split them into many adhoc tasks and run in parallel.

Could be individual tasks or better batches like 100 or 1000 events per adhoc task.