do not run didReceiveRemoteNotification and performFetchWithCompletionHandler simultaneous

[r10s]

it happens from time to time that, for whatever reason, didReceiveRemoteNotification and performFetchWithCompletionHandler are called at nearly the same time (only very few milliseconds in between).

that results in duplicated called to start_io, maybe_network, stop_io - at least the latter always from the main thread,

that may lead to some database and core stressing leading to issues as #1129 at the end.