core: perform retries even without initial connection

In https://github.com/lyft/envoy-mobile/issues/696, we did some testing and determined that **when launching the library offline, Envoy Mobile does not retry requests** until it has established an initial connection. This is likely due to the fact that retries are performed by the router, and outbound requests don't get that far in this case.

Fixing this issue would likely non-trivial due to the fact that retries are performed by the router independently of Envoy Mobile or client state, but is something we should investigate.