fix: remove inline GC and schedule a background task instead (#1610)

* fix: remove inline GC and set a default value of gc interval

- remove inline GC
- add a default value of GC interval
- run the GC periodically by default with the default value if no interval provided
- generate GC tasks with a random delay(0-30s) between
- add IsReady() method to scheduler.TaskGenerator interface

Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

* ci: add test for gc with short interval

Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

---------

Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

examples/config-conformance.json

@@ -2,8 +2,8 @@
   "distSpecVersion": "1.1.0-dev",
   "storage": {
     "rootDirectory": "/tmp/zot",
-    "gc": false,
-    "dedupe": false
+    "gc": true,
+    "dedupe": true
   },
   "http": {
     "address": "0.0.0.0",