QtGrpc: improve relation between (client) operation (context) and channel

The proxy object "QGrpcOperationContext" which the internal Http2Handler uses to communicate with the RPC operation has some lifetime issues in combination with its corresponding (loose) Http2Handler counterpart.

Currently, for code like the following:

QCOMPARE_NE(streamPtr, client()->testMethodServerStream(request).get()); 

The right entry is creating and immediately destroying the object. In case of a active connection the current implementation will continue to process the request even though nothing to respond to is alive!

We need to improve the channel implementation to keep track of such cases (e.g. flagging handlers as dirty / dead) and do some cancellation.