Add exponential backoff in rpc done polling code

Right now there is a fixed polling interval of `1 ms`, however for long running rpcs this will be inefficient. There should be an exponential backoff in the polling interval based on the number of polling attempts made starting at 1 ms and backing off so some reasonable value like `100 ms` so that short RPCs are not delayed but long rpcs do not get overpolled:

https://github.com/iotile/coretools/blob/master/transport_plugins/jlink/iotile_transport_jlink/jlink_background.py#L125