Gather .health instance stats

Reworks instance checks to use the same timestamp and be transactional. Also prevents long DB lock times during list updates.

server/templates/admin.html.j2

@@ -16,7 +16,7 @@
     
     <h4>Instances</h4>
     {% if is_admin %}<p class="text-danger-emphasis">root mode</p>{% endif %}
-    <div><canvas id="graph" width="600" height="200"></canvas><noscript>History feature works only with javascript enabled.</noscript></div>
+    <div class="table-responsive"><canvas id="graph-health" class="table" height="400"></canvas><noscript>History feature works only with javascript enabled.</noscript></div>
     <div class="container mt-5">
       <form>
           <div class="row">