EZC: fix crash due to EG(exception) being transmitted between requests

Summary: When an exception was thrown from EZC, EG(exception) was left holding a
reference to the smart-allocated exception object, even after the end of
the request. At the end of the request, the object storage was freed by
resetAllocator(), then when zend_wrap_func() was entered in the next
request, EG(exception) would be freed again, causing a crash.

Fixed by installing request event handlers. The requestInit() handler is
analogous to Zend's init_executor().

Closes https://github.com/facebook/hhvm/pull/3121

Reviewed By: ptarjan, joelm

Differential Revision: D1435383

Pulled By: @swtaarrs