Agent task queue pop rate limit

[DavidePrincipi]

Under unexpected conditions, the agent task queue may be filled with hundreds of tasks. The agent has no rate limit when it pops the tasks out of its queue: this behavior might lead to high loads and OOM killings.

Reported by @NethNick with hundreds of get-cluster-status tasks in the queue.
Discussion https://mattermost.nethesis.it/nethesis/pl/nuuxze8e6fyi8fx3xg88hn87mr

Relevant agent code: https://github.com/NethServer/ns8-core/blob/005f7bc89077c8e598e4cdb7fbf949cb522e878c/core/agent/htask.go#L345

[DavidePrincipi]

Agent overload error

[DavidePrincipi]

In Testing core 3.13.0-dev.3

Test case

Reproduce an heavy cluster load condition by quickly click on Cluster Status and Nodes page tenths of times.

Once updated to the Testing version, check that NS8 agents start to drop new task requests with more than 32 simultaneous running tasks and the node load is limited.