Make each server thread own and destroy its EventBase
Make each server thread own and destroy its EventBase

Summary: This ensures that EventBases are destroyed by server threads, so that all destruction callbacks are run in those threads as well.

Reviewed By: jmswen

Differential Revision: D4644793

fbshipit-source-id: 200d46bcd661d69c4aa6d84b470a3344954d00e7

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter