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`.
Because now we're limiting the range we're reading from view building
tasks table, we need to make sure that new tasks are created with larger
uuid then the `min_alive_uuid`.
In order to do it, we need to be able to see current `min_alive_uuid`
while creating new tasks.
After previous commits, we can drop entire task's state and replace it
with single boolean flag, which determines if a task was aborted.
Once a task was aborted, it cannot get resurrected to a normal state.
The coordinator is responsible for building tablet-based views.
It schedules tasks for `view_building_worker` and updates views'
statuses.
The tasks are scheduled in a way that one shard is processing only one
tablet at most (there may be multiple tasks since a base table may have
multiple views).
Support for tablet operations will be added in next commits.
The worker is responsible for building tablet-based views by
executing tasks scheduled by the view building coordinator.
It observes view building state machine and wait on the machine's
conditional variable (so the worker is woken up when group0 state is
applied).
The tasks are executed in batches, all tasks in one batch need to have
the same: type, base_id, table_id. One shard can only execute one batch
at a time (at least for now, in the future we might want to change
that).
That worker keeps track of finished and failed tasks in its local state.
The state is cleared when `view_building_state::currently_processed_base_table`
is changed.
`view_building_state` holds mapping of `view_building_task`s for
tablet-based views. The structure is a memory representation of data
stored in group0 tables.
`views_state` holds information about tablet-based views and their
build status.
