wifi scan while connected doesn't work and wifi mode switching problems

[rtrue983]

there are at least 2 issues i see with the wifi library right now

1. when turning off the AP during runtime by calling mgos_wifi_setup_ap( wifi.ap.enable=false ) the STA will sometimes now connect because the esp radio isn't started
2. wifi_scan does not handle the scenario where STA is connected and needs to be stopped (similar to how it handles the STA being off
3. similar to 1 if STA is connected and AP is turned off, STA will sometime disconnect and have to reconnect
4. calling wifi scan will sometimes not produce a scan callback

To elaborate on 3, if current mode is AP+STA, STA is connected to a network... there needs to be a good way to just turn the AP off without causing a STA Disconnect. similarly when turning AP on.

This seems to fix item 4, but probably needs broader testing