hmon: use single Arc for heartbeat monitor

[arkjedrz]

Replace previous approach with two Arcs.

[github-actions]

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[License Check Output]
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INFO: Analyzed target //:license-check (169 packages loaded, 10409 targets configured).
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INFO: Elapsed time: 25.943s, Critical Path: 2.55s
INFO: 16 processes: 12 internal, 3 processwrapper-sandbox, 1 worker.
INFO: Build completed successfully, 16 total actions
INFO: Running command line: bazel-bin/license.check.license_check ./formatted.txt <args omitted>
usage: org.eclipse.dash.licenses.cli.Main [-batch <int>] [-cd <url>]
       [-confidence <int>] [-ef <url>] [-excludeSources <sources>] [-help] [-lic
       <url>] [-project <shortname>] [-repo <url>] [-review] [-summary <file>]
       [-timeout <seconds>] [-token <token>]

[github-actions]

The created documentation from the pull request is available at: docu-html

[danth]

Previously, calling get_eval_handle a second time for the same monitor would return a new handle with a separate timestamp. After this change, the same timestamp is shared between all handles.

I'm not sure what the expected behaviour should be, but it would be good to add a test for this either way.

[arkjedrz]

@danth, thanks for taking a look.
Each monitor should have get_eval_handle used once - by HealthMonitor::get_monitor, so the issue would never be observed using public APIs. This method is returning handles to the same monitor, so it was not a right choice to have a stateful handle in the first place. I added a test case for that, it is not passing for main branch.
BTW the context of this change is to enable protected memory allocator, having a double Arc unnecessarily complicates things.