ios: use internal engine thread (#530)
ios: use internal engine thread (#530)

Recent changes (namely https://github.com/lyft/envoy-mobile/pull/498) made it unnecessary for the platform-level clients to start Envoy on newly spawned threads. In fact, these changes actually introduced potential races where consumers of the platform clients could end up calling `send` on engines that hadn't finished starting on these threads.

This PR removes the custom threading logic in Swift in favor of simply using the new threading being done within the core. Another PR will make the same change on Android: https://github.com/lyft/envoy-mobile/pull/531.

Signed-off-by: Michael Rebello <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@michaelrebello.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