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>

test/blackbox/scrub.bats

@@ -61,7 +61,7 @@ function teardown() {
     wait_zot_reachable "http://127.0.0.1:8080/v2/_catalog"
 
     # wait for scrub to be done and logs to get populated
-    run sleep 10s
+    run sleep 15s
     run not_affected
     [ "$status" -eq 0 ]
     [ $(echo "${lines[0]}" ) = 'true' ]
@@ -76,7 +76,7 @@ function teardown() {
     wait_zot_reachable "http://127.0.0.1:8080/v2/_catalog"
 
     # wait for scrub to be done and logs to get populated
-    run sleep 10s
+    run sleep 15s
     run affected
     [ "$status" -eq 0 ]
     [ $(echo "${lines[0]}" ) = 'true' ]