swift: rename EnvoyRunner to RunnerThread (#276)
swift: rename EnvoyRunner to RunnerThread (#276)

I think this is a bit clearer as to what it functionally does, and this is already within the `Envoy` namespace (`Envoy.RunnerThread`).

Signed-off-by: Michael Rebello <d207f8179edebd36c6ff33f7db1973b360eda8cd@lyft.com>

envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile