Fixed memory leak

• Bugfix: There was an important heap memory leak when more than two nodes were connected. All message queues use static memory now.
• Improvement: Clock time is now managed by internal SDK time subsystem
• New Feature: Change default filesystem to LittleFS on ESP8266 platform. Notice that in case of a filesystem change device will lose its configuration. You can reconfigure normally using its web UI by connecting to its AP
• New feature: Add method to reboot gateway from REST API
• Bugfix: There was a bug in case a node is programmed to sleep forever
• Bugfix: JSON strings use double quotes now
• Compilation improvements for Platformio
• Some bugfixes and code clean up