grpc-js: Defer actions in http2 stream write callback

[murgatroid99]

nodejs/node#49147 shows that in some rare scenarios, calling end in the write callback can put the whole session into a bad state, where the socket is closed but the session is treated as still open. This could cause the symptoms described in #2502. nodejs/node#49147 (comment) indicates that this problem does not occur if the contents of the write callback are enclosed in process.nextTick, so that's what I did here. In case stream.close in the write callback can cause the same problem, all of the code in the callback is enclosed in process.nextTick. This callback is already always asynchronous with respect to other code, so other than avoiding this bug, this change should have no observable effects to user code.