Features/async sender and queue

I've added an async channel option (implemented via an async queue and sender)

To replace the sync channel that is there by default:

sender = ApplicationInsights::Channel::AsynchronousSender.new
queue = ApplicationInsights::Channel::AsynchronousQueue.new sender
channel = ApplicationInsights::Channel::TelemetryChannel nil, queue
tc = ApplicationInsights::TelemetryClient.new channel