ESPiLight.cpp: replace pilight json generation.

[janLo]

The pilight jdon generation has the problem that it has a buffer that rets reallocated with twice the size every time it is too small. This causes the ESP to crash if we're <10k of free memory in some cases.

So this replaces this by a simple hand-written json generation that uses in advance how much space it needs.

[puuu]

You reserve one byte to much, because of the missing , of the first element. But accepted.

[puuu]

Thanks! Merged in 6088666793c1ce5ff944e4f01c48b1d18a7bf94d . New release is on the way.