Release candidate nine for v1.0.0 release

Release candidate with a number of improvements and bug fixes:

• Optional attitude_estimator_q from Anton Babushkin (@DrTon, lightweight, elegant, estimates gyro offsets)
• New mixer: Modifies collective throttle to keep system stable when motors saturate (by @Tumbili)
• Preflight checks: Prevent takeoff with unhealthy system configurations (by @Zefz)
• Logging: Default to 100 Hz on Pixhawk (user-configurable via parameter)
• MAVLink FTP: 30x speedup and bugfixes (by @DonLakeFlyer)
• Improvements for gyro, mag and accel calibration (more rigour but also more user assistance)
• MPU6K, LSM303D and L3GD20(H): Reject duplicate measurements (by @tridge Andrew Tridgell)
• Better boot error reporting (by @sjwilks)
• Fix airspeed handling in ECL fixed wing controller (by @thomasgubler)
• Firefly Y6 support (by @Tumbili)
• Only transmit used parameters feature + fixes, speeds up param transfers (by @davids5)
• Fixes for state-level HIL
• Fixes for offboard control with velocity commands
• Param meta data by @DonLakeFlyer
• STM32F4Discovery support (by @nopeppermint)
• Various mavlink fixes (by @NaterGator)