Improve board responsiveness when MQTT broker is up but backend is down.

[PBrunot]

This is probably a corner case, configuration error on the backend side, but it makes the board almost unusable.

[github-actions]

Firmware size changes for esp32-devboard

Commit CURRENT 099afe7 vs REFERENCE e3bd43d

<CURRENT> MAP file: ../firmware.map.latest
<REFERENCE> MAP file: ../firmware.map.previous
Difference is counted as <CURRENT> - <REFERENCE>,           i.e. a positive number means that <CURRENT> is larger.
Total sizes of <CURRENT>:                                       <REFERENCE>     Difference
Used static DRAM:   53636 bytes (  70944 remain, 43.1% used)          53620            +16 (    -16 remain,      +0 total)
      .data size:   21004 bytes                                       21004
      .bss  size:   32632 bytes                                       32616            +16
Used static IRAM:   69782 bytes ( 102250 remain, 40.6% used)          69782                (     +0 remain,      +0 total)
      .text size:   68755 bytes                                       68755
   .vectors size:    1027 bytes                                        1027
Used Flash size :  919591 bytes                                      918819           +772
           .text:  787571 bytes                                      786895           +676
         .rodata:  131764 bytes                                      131668            +96
Total image size: 1010377 bytes (.bin may be padded larger)         1009605           +772

[github-actions]

Firmware size changes for hardware-rev0-it_IT

Commit CURRENT 099afe7 vs REFERENCE e3bd43d

<CURRENT> MAP file: ../firmware.map.latest
<REFERENCE> MAP file: ../firmware.map.previous
Difference is counted as <CURRENT> - <REFERENCE>,           i.e. a positive number means that <CURRENT> is larger.
Total sizes of <CURRENT>:                                       <REFERENCE>     Difference
Used static IRAM:   16383 bytes (      1 remain, 100.0% used)          16383                (     +0 remain,      +0 total)
      .text size:   15356 bytes                                       15356
   .vectors size:    1027 bytes                                        1027
Used stat D/IRAM:   83379 bytes ( 262477 remain, 24.1% used)          83363            +16 (    -16 remain,      +0 total)
      .data size:   16448 bytes                                       16448
      .bss  size:   31392 bytes                                       31376            +16
      .text size:   35539 bytes                                       35539
Used Flash size :  946823 bytes                                      946099           +724
           .text:  810667 bytes                                      810039           +628
         .rodata:  135900 bytes                                      135804            +96
Total image size: 1015193 bytes (.bin may be padded larger)         1014469           +724