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

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().
Sure, assuming no other shutdown handler sets it again after it is cleared. I don't think there's much overhead -- the virtual call will be done even if we don't implement a requestInit() handler here. Maybe m_data.prev_exception should be wiped as well, to be on the safe side.