Set constructor of JS hub proxies
- #3210
PortableDroid - updating references to Microsoft.Net.Http NuGet package

Updating references to Microsoft.Net.Http NuGet package to fix build errors appearing when building a Xamarin Android project after adding the Signalr Client NuGet package to the project.
Disable keep alive checks in long polling transport - JS
Timeout long polls exceeding the ConnectionTimeout on the JS client
Disable keep alive checks in long polling transport - .NET
Merge branch 'release' into dev
OnStartedTouchedBase - moving OnStarted to ClientTransportBase

All client transports have to follow the same start pattern where they have to override the OnStarted method instead of the Start method.
WhoFliesHighFallsLow - pushing TransportInitializeHandler to ClientTransportBase

Removing references to TransportInitializationHandler from transport implementations since initialization is now handled in the ClientTransportBase class. Types derived from the `ClientTransportBase` can subscribe to the OnStartFailed event to be notified if/when a failure occurs during starting the transport and can invoke the `TransportFailed` method if a transport specific error occurred to notify `ClientTransportBase` about the error.
Addressing review comments