Only store channel if use_cached_channel is true (#387)

The gRPC client exposes a parameter that disables reusing a cached
channel: use_cached_channel. Setting this to false doesn't stop the
channel (and mock) from being inserted to the internal channel cache. 
The problem with it being inserted to the map means the channel will
only get destroyed once it's being replaced by another channel.

In our case we want the channel (and associated connection) to be closed
 as soon as the client using it is getting destroyed though, meaning we don't
 want to reuse the channel at all. I believe this is also consistent with the 
description for use_cached_channel:

If false, a new channel is created for each
new client instance. When true, re-use old channels from cache for new
client instances. The default value is true

src/c++/library/grpc_client.cc

@@ -135,12 +135,15 @@ GetStub(
       grpc::CreateCustomChannel(url, credentials, arguments);
   std::shared_ptr<inference::GRPCInferenceService::Stub> stub =
       inference::GRPCInferenceService::NewStub(channel);
-  // Replace if channel / stub have been in the map
-  if (channel_itr != grpc_channel_stub_map_.end()) {
-    channel_itr->second = std::make_tuple(1, channel, stub);
-  } else {
-    grpc_channel_stub_map_.insert(
-        std::make_pair(url, std::make_tuple(1, channel, stub)));
+
+  if (use_cached_channel) {
+    // Replace if channel / stub have been in the map
+    if (channel_itr != grpc_channel_stub_map_.end()) {
+      channel_itr->second = std::make_tuple(1, channel, stub);
+    } else {
+      grpc_channel_stub_map_.insert(
+          std::make_pair(url, std::make_tuple(1, channel, stub)));
+    }
   }
 
   return stub;