system: refactoring & delays clean-up

- BREAKING replace loadavg and system check from millis to seconds
- implement all available system clocks, make sure it is correctly
  scaled with the origin type
- fix duration::{millis(),ccount(),seconds()} and return a respective
  clock's time_point instead of duration. just like led sources, make
  sure every module stores timestamps as time_point and intervals as
  duration
- no need to track millis overflow on top of existing overflow counter
  of the micros64 in getUptime
- remove class overhead in led static delays conversion, just try to
  inline the required calculations and use NAME as unique id

also fix some non-apparent issues with heartbeat
- allow zero interval when it is also Mode::Once
- iterating runners to find the interval might return zero

code/espurna/crash.cpp

@@ -240,7 +240,7 @@ extern "C" void custom_crash_callback(struct rst_info * rst_info, uint32_t stack
     }
 
     // Do not record crash data when doing a normal reboot or when crash trace was disabled
-    if (checkNeedsReset()) {
+    if (pendingDeferredReset()) {
         return;
     }
 
@@ -297,7 +297,7 @@ void crashDump(Print& print) {
 }
 
 void crashResetReason(Print& print) {
-    auto reason = customResetReason();
+    const auto reason = customResetReason();
     bool custom { CustomResetReason::None != reason };
     print.printf_P(PSTR("last reset reason: %s\n"), custom
         ? customResetReasonToPayload(reason).c_str()