Prevent concurrent requests when reassigning/delegating a task

Theoretical steps to reproduce:

1. Create 2 users jack and john and add them to the "members" group
2. As any user, create a Note document
3. Then start a Serial Review workflow on this Note document
4. Then assign the first task "Choose Participants" to the "members" group and press the "Start review" button
5. Now, with 2 different browsers, log in as john and jack
6. Navigate to the task which is assigned to you
7. Reassign the task to you :
   • select your user in the "Choose the user" field
   • at the exact same time, push the button Reassign

Observed behavior:

• the current task is assigned to john : you can see in the task metadata that the "nt:actor" field contains "john"
• the Everything permission is assigned to jack : you can see the permission in the Task ACLs
• in the nt:taskComments metadata, you can see 2 values, one for jack, one for john
• because of this configuration, the task cannot be processed any more: the current assignee does not have the permission to manage the task

Expected behavior:
The reassign method prevents from running its code by 2 concurrent users. It could use the same mechanism as the "endTask" method which uses LockHelper#doAtomically

PS: it's difficult to observe this bug but it has happened already twice at the customers'