i#297 sideline thread exit: split synch-all op into the process exit

Created by: zhaoqin

We split the synch-all threads operation into two synch operations:

• pre-exit synch: synchronize all app threads and ignore client threads
• post-exit synch: synchronize all threads. The pre-exit synch is called before all client thread exit and process exit events. The post-exit synch is called after all the client exit events. By doing so, the sideline client thread could have a chance to perform graceful exit from the process exit event.

Fixes #297