Safer Clients

Summary:
Python is considered a Safe Enivronment, where things like aborts and segfaults
can not happen.

If clients where used outside of "async with" we use to print a error message
and then C++ would abort.

This fix makes sure C++ no longer aborts but we still print the message
that the "async with" should be used.

All futures are now shielded so cancel will not produce strange exceptions when
C++ decides to fullfill the canceled future.

Also we now pass a tuple of client and future to all bridgeFutureWith calls.
This has the affect of making sure the client can not GC before the future
is completed.

We now use a unique_ptr for RequestChannel so there is no more race to delete
the shared pointer.

Reviewed By: yfeldblum

Differential Revision: D7389380

fbshipit-source-id: 6a3ca019190907e726c550c9b379a2a2610d0b0a

folly/python/__init__.pxd

@@ -13,7 +13,7 @@ cdef extern from "folly/Try.h" namespace "folly" nogil:
 
 cdef extern from "folly/futures/Future.h" namespace "folly" nogil:
     cdef cppclass cFollyFuture "folly::Future"[T]:
-        pass
+        T get()
         #TODO add via and then
 
 cdef extern from "folly/Unit.h" namespace "folly":