HttpClient fix for all locators

[Indigo744]

Continuing #198 and the issue found by @d8ahazard

This PR brings the following fixes:

• use a shared HttpClient to avoid socket exhaustion
• dispose of HttpResponseMessage
• Add discovery test with all locators running in parallel

The rational to use a shared HttpClient is from:

• https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/
• https://stackoverflow.com/questions/15705092/do-httpclient-and-httpclienthandler-have-to-be-disposed
• Need Official Guidance On using HttpClient in Functions Azure/azure-functions-host#1806

This is actually the way the HueClient class is built, with either a shared HttpClient, or a given one by the user in the constructor.

I choose not to change the IBridgeLocator signature for now and use a local common HttpClient. But if you want me to add a new constructor signature to match the HueClient class, feel free to ask.