RSC15f: implement fallback affinity

[paddybyers]

This PR implements temporary affinity to a fallback host for REST operations, as specified in https://docs.ably.io/client-lib-development-guide/features/#RSC15f:

(RSC15f) Once/if a given fallback host succeeds, the client should store that successful fallback host for ClientOptions.fallbackRetryTimeout. Future HTTP requests during that period should use that host. If during this period a qualifying errors occurs on that host, or after fallbackRetryTimeout has expired, it should be unstored, and the fallback sequence begun again from scratch, starting with the default primary host (rest.ably.io or ClientOptions#restHost)

Affinity to a successful fallback host is for 10 minutes by default, but is configurable via the fallbackRetryTimeout client option as specified in https://docs.ably.io/client-lib-development-guide/features/#TO3l10.