Fix "Seyren webapp shutdown stuck on CheckScheduler.executor's non terminated thread".
1. Add `@PreDestroy` hook to shutdown `CheckScheduler.executor`
2. Make `CheckScheduler.executor` threads `daemon`
Fix "Seyren webapp shutdown stuck on CheckScheduler.executor's non terminated thread".
1. Add `@PreDestroy` hook to shutdown `CheckScheduler.executor`
2. Make `CheckScheduler.executor` threads `daemon`
Merge pull request #75 from cyrille-leclerc/master

Seyren webapp shutdown stuck on CheckScheduler.executor's non terminated thread