Simple BLE: add function to change connection interval

[bradjc]

Would be useful

[brghena]

Waiting seems to have solved this problem for us. SDK 10 introduced ble_conn_params_change_conn_params(...)

"Use this function to change the connection parameters to a new set of parameter (ie different from the ones given at init of the module). This function is usefull for scenario where most of the time the application needs a relatively big connection interval, and just sometimes, for a temporary period requires shorter connection interval, for example to transfer a higher amount of data. If the given parameters does not match the current connection's parameters this function initiates a new negotiation."

I'll leave the issue open until one of us actually tests it.