ESP8266: Allow auto modem sleep if power off and sleep permitted

[pfeerick]

Closes #1144.

Adds a 1ms delay only when main power is off and the noWifiSleep option is not active, and only for the ESP8266. This is needed for the automatic modem sleep to operate on the ESP8266 between DTIM beacons. The ESP32 appears to have a different mechanism, via the esp_wifi_set_ps() API call, but that a problem for another issue/PR.

I'm leaving this as a draft for now, until I've been able to test it for a few days for unforeseen side effects. ;)

To show the effect of this on my Wemos D1 Mini, I powered it via USB and put a calibrated power meter on it. The LED Strip and relay is still being powered via the main PSU.

Pre PR video - ~150ma continous

Post PR - rolling 90-120ma

Post PR - rolling 80-140ma - second meter

[Aircoookie]

Thank you :) Let me know once you believe it is ready for review!

[pfeerick]

I think it's time to call it cooked. Just short of 5 days uptime with zero stability issues. Driving a 507 led strip in four segments, with static and animated effects run each day and being poked by Home-Assistant for notifications, this seems to be mostly working - sometimes it seems to still get hung up in 'full power' mode, but if left alone long enough seems to revert to sleepy modem mode. Either that, or perhaps unticking "Disable WiFi sleep" doesn't always work without a restart (which would not surprise me given how... unpredictable... oh, alright, downright buggy... the ESP8266 API is).

I'll look as spinning up a ESP32 and see if I can see what's happening over there re: modem sleep, etc. I'll probably poke around the guts of WLED a bit more and see what's keep the ESP8266s attention, but I suspect it's the amount of network traffic / something on the network waking it up, not WLEDs fault.