DCHECK on reentrant invocations of loop()

DCHECK on reentrant invocations of loop()

Summary:
I had a crash bug where an `EventBase` handler called a function, which tried to use the thread's EventBase as if it were its own:

```
auto evb = folly::EventBaseManager::get()->getEventBase();
// schedule work on evb, which calls evb->terminateLoopSoon() on completion
evb->loopForever();
```

This ended up invoking the `event_base_loop()` reentrantly, and corrupting the heap in a hard-to-debug way.

Reviewed By: djwatson

Differential Revision: D3408155

fbshipit-source-id: b8855aa3b390fa33e032ab295d386122d2cb872a