Reduce memory footprint

[miceno]

This PR provides a heavy optimization over the strings used for text sensors, moving them from main SRAM memory to PROGMEM. In order to do it, it allocates static buffers for each text sensor up to the size of the max length of the strings. Although it requires static allocation, it prevents memory fragmentation and allow freeing a significant amount of memory.

In addition it includes an optimization on the management of the loop to show stats when the loop is taking too long.

And it also solves a recurrent "Too old data" issue by updating last_transmission when data is not available.

[syssi]

Thanks for your contribution! Could you provide a comparison of the memory footprint (previous & now)? Thanks in advance!

[miceno]

I measure free heap and max heap block and here it is the result:

branch	free heap	max heap block
main	23832	19792
feat/reduce-memory	29344	24888

It is a memory reduction of over 5000 bytes.

[miceno]

Hold on, since I am experiencing memory leaks. It needs further research.

[miceno]

I forgot to update... it is was a false alarm, my devices was restarting from time to time and I thought it was the new code, but it wasn't that 👍

Please, review it and let me know if the PR requires some refinement.