[reconfigurator] background task for marking VMMs to be stopped for update - #11170

control: job assigned to worker 01M113V8TKX6DYCHF2V99TMD7K [factory aws, i-066131607a2ce1a81] (queued for 1 m 0 s)
...
task: starting task 0: "setup"
task: process exited: duration 4113 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 15871 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 17 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 13980 ms, exit code 0
...
task: starting task 4: "build"
...
| a out/probe.tar.gz 3037K
| a out/propolis-server.tar.gz 34642K
| a out/pumpkind-gz.tar 26647K
| a out/switch-softnpu.tar.gz 504508K
| 
| real     1:29.529185826
| user     1:26.562296946
| sys         4.978837123
| trap        0.006483014
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:26.439097094
| lat         1.067090713
| stop        0.000174470
task: process exited: duration 1114367 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (2776726849 bytes)
|W| uploaded: /work/package.tar.gz