Stop CxxBridge run loop when bridge is invalidated

Summary:
This fixes the `testModulesAreDeallocated` test. It was sometimes failing because the bridge's module was still alive. I debugged with the Xcode memory graph and found that the bridge was actually kept alive by a pointer from the NSThread.
By killing the runloop on the thread, the thread will eventually die and thus free the bridge.

One thing I didn't investigate was whether the thread was alive because of it receiving actual messages or just because the run loop was spinning.

Reviewed By: javache

Differential Revision: D5729925

fbshipit-source-id: 304f526129d2c5e137bfd791a6f957f6169b783e

React/CxxBridge/RCTCxxBridge.mm

@@ -962,6 +962,10 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
       self->_moduleDataByID = nil;
       self->_moduleClassesByID = nil;
       self->_pendingCalls = nil;
+
+      [self->_jsThread cancel];
+      self->_jsThread = nil;
+      CFRunLoopStop(CFRunLoopGetCurrent());
     }];
   });
 }