Add a new Instance.health_check_started field

This will enable us to provide more useful information for the user,
now that all user-triggered health checks are async.

Also, de-bounce the health check endpoint to not allow additional
health check tasks to be triggered when one is already in progress.

awx/main/migrations/0171_add_health_check_started.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.13 on 2022-09-26 20:54
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0170_node_and_link_state'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='instance',
+            name='health_check_started',
+            field=models.DateTimeField(editable=False, help_text='The last time a health check was initiated on this instance.', null=True),
+        ),
+    ]