Bugfix/esp crashes

[sarfata]

Improve stability of ESP module.

Several fixes:

• A hardware watchdog timer crash that would happen very regularly under load (every 10/15 minutes) caused by a bug in the ESP8266 Arduino framework and which requires that we use the staging version for now (see Fix for #4565 (rx fifo length), protect access to rx_buffer esp8266/Arduino#4568 for more info)
• Crashes with an Exception (29) which seemed to be related to our TCP server code. We were using a linked list of messages left to send which could easily get out of control. Replaced that with a circular buffer.
• Also closes Make the ESP LED turn green when a WebSocket client is connected #96

Done a lot of testing with multiple clients connected via TCP and WebSocket and not seeing crashes anymore for now.

[coveralls]

Coverage remained the same at 66.406% when pulling 48ee98b on bugfix/esp-crashes into c041cb3 on master.