Make jira cache flush events non-blocking

Problem Definition

If in some case when many cache flush events are generated (see JRA-64839) all those events are blocking events. During that time cache is not usable and if this is hot cache that degrades JIRA performance.

Following sequence will happen:

1. Cache flush events generated in loop - Thread1
2. Cache will be flushed - Thread1
   • If this is a hot cache, there will be attempt to populate it - Thread2
   • Thread will try to flush cache but will be blocked - Thread1
3. Cache population is done - Thread2
4. Cache will be flushed - Thread1
5. Repeat until all cache events are done.

Suggested Solution

Make jira cache flush events non-blocking

JRASERVER-67019 makes cache flush less coupled for DC.

Workaround

None