Fix a couple QuicTransportBase bugs
Fix a couple QuicTransportBase bugs

Summary:
1) unregistering a pending write callback from another callback was unsafe.  Follow the paradigm in the rest of this function.
2) Calling closeGracefully() nulls out the connection callback but keeps writing -- need to check that connCb_ is non-null before calling onAppLimited()

Reviewed By: kvtsoy

Differential Revision: D40811191

fbshipit-source-id: fa3512e4aa5ddda3dfc160a044b7d29b7aec8a8a

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst