Fix bigtable channel refreshing for DataConnection

[dbolduc]

We use a temporary shared_ptr<CompletionQueue> when we create the Stubs.

google-cloud-cpp/google/cloud/bigtable/internal/bigtable_stub_factory.cc

Line 60 in 18937be

auto cq_ptr = std::make_shared<CompletionQueue>(cq);

So DataConnection never refreshes its channels. It always backs out, because there are no more references to the CQ:

google-cloud-cpp/google/cloud/bigtable/internal/connection_refresh_state.cc

Lines 74 to 75 in 18937be

	auto cq = weak_cq.lock();
	if (!cq) return;

We can fix the problem by updating the channel refresh API to accept a shared_ptr<internal::CompletionQueueImpl> instead of a shared_ptr<CompletionQueue>. The CQImpl is also owned by the DataConnection's background threads, so it will not go out of scope until the class goes out of scope.