ADSBee 1090 Firmware 0.9.0 Release Candidate 19

This PR provides significant improvements to the Mode S demodulator and network uplink firmware stacks, allowing the ADSBee to process more Mode S messages per second (especially squitter packets), and process / upload them as necessary without overflowing internal queues. Users should notice a lot more squawk codes getting filled in, and higher packet rates in high traffic areas.

• Broaden the net for messages fed into the demodulator (accept incorrect word count values from demodulator in case it didn't stop in time).
• Add early flushing to packet reporting queues to prevent overflows that were dropping packets.
• Improve decoding of aircraft operational status messages on Mode S.
• Fix incorrect decoding of operational status messages that was showing aircraft as on ground.
• Improve labelling of handshake timeout messages for better diagnostics.
• Add serial logger script for monitoring multiple devices with coherent timestamping.
• Fix Mode S emitter category reported over GDL90.
• Update README (thanks @aymzhang )!