Unwrap AggregateException when using a synchronous method in ChannelPool.
Unwrap AggregateException when using a synchronous method in
ChannelPool.

This will fix https://github.com/GoogleCloudPlatform/google-cloud-dotnet/issues/206

jskeet/gax-dotnet,jskeet/gax-dotnet,googleapis/gax-dotnet,googleapis/gax-dotnet