Fix wraparound bug in backlog

[emontnemery]

Description:

Fix wraparound bug in backlog caused by assuming that millis time 0 would always be in the past.
After around 25 days of uptime, millis() passed 2^31 and the backlog stopped working.

Also rename backlog_delay to backlog_timer + fix a similar (but more harmless) assumption in wifiKeepAlive

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works on Tasmota core ESP8266 V.2.7.4.7
• The code change is tested and works on Tasmota core ESP32 V.1.0.4.2
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[effelle]

Tested on ESP32. All fine.