`cluster.routing.allocation.cluster_concurrent_rebalance` setting,
clarifying in which shard allocation situations the rebalance limit
takes effect.
Closes#20529
Today we throttle recoveries only for incoming recoveries. Nodes that have a lot
of primaries can get overloaded due to too many recoveries. To still keep that at bay
we limit the number of threads that are sending files to the target to overcome this problem.
The right solution here is to also throttle the outgoing recoveries that are today unbounded on
the master and don't start the recovery until we have enough resources on both source and target nodes.
The concurrency aspects of the recovery source also added a lot of complexity and additional threadpools
that are hard to configure. This commit removes the concurrent streamns notion completely and sends files
in the thread that drives the recovery simplifying the recovery code considerably.
Outgoing recoveries are not throttled on the master via a allocation decider.
Aron Szanto
Herman Schaaf
Boaz Leskes
Ali Beyad
Clinton Gormley
Simon Willnauer
Yannick Welsch