Add a new cluster parameter for the maximum number of concurrent resharding operations that are allowed.

Write to sharding_cluster_parameters.idl which is for cluster-topology state such as shard cardinality and add/remove-shard in progress. We want all shards to agree on the cap (wouldn't be safe for one shard to think maximum is N and another shard to think the maximum resharding operations allowed is M).

The default value of the cap should be 10.

This ticket should land after SERVER-138049 land, so that we don't have to worry about it being some function of the max thread count.

Since this ticket is pulling the cap out, and thus eliminating the dependency on the server parameters for max thread count (per coordinator, donor, and recipient) which was made safe by SERVER-138049, this ticket should also restore the default server parameter maxThreadCount values for coordinator to 5, donor to 2, and recipient to 8.