With the new `min_alive_uuid` saved in the group0 table,
we need to make sure that all new tasks are created with time uuid
greater than the value saved in `min_alive_uuid`.
This patch introduces the `task_uuid_generator` which ensures that
when we are generating multiple tasks in one group0 command, each task
will have an unique time uuid and each time uuid will be greater than
`min_alive_uuid`.
b64f2d2e90