Add a service::topo::global_topology_request_virtual_task, which
covers the replication factor changes.
Currently, the global_topology_request_virtual_task can be aborted
only if it is paused.
The progress of the rf change isn't counted.
Extend tablet_virtual_task::wait to support resize tasks.
To decide what is a state of a finished resize virtual task (done
or failed), the tablet count is checked. The task state is set to done,
if the tablet count before resize is different than after.
Extend tablet_virtual_task::wait to support migration tasks.
To decide what is a state of a finished migration virtual task
(done or failed), the tablet replicas are checked. The task state
is set to done, if the replicas contain the destination of a tablet
migration.
