client: fix client blocking during garbage collection

[mismithhisler]

Description

This change removes any blocking calls to destroyAllocRunner(), which caused nomad clients to block when running allocations in when shutdown_delay was enabled at the group level. In addition, this change consolidates client GC by removing the MakeRoomFor() method, which is redundant to keepUsageBelowThreshold().

Testing & Reproduction steps

The trick to reproducing this issue is getting MakeRoomFor to trigger a GC. This can be done quickly via client settings of:

gc_max_allocs = 2 (or some low number)
gc_parallel_destroys = 1

MakeRoomFor contends with another method for triggering GC which is AllocStateUpdated, and normal GC timers. Firing periodic jobs every second can get MakeRoomFor to pop an alloc for GC.

job "hello-world" {
  type = "batch"
  
  periodic {
    cron             = "* * * * * * *"
    prohibit_overlap = true
  }

  group "servers" {
    // this at the *group* level is very important for triggering this behavior
    shutdown_delay = "1m"

    task "test" {
      driver = "docker"

      config {
        image   = "alpine:latest"
        command = "/bin/sh"
        args = ["-c", "echo", "123"]
      }

      resources {
        cpu    = 50
        memory = 64
      }
    }
  }
}

Links

Fixes GH #19917

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