Identify several landing & takeoff events for same flight_id

[Xenotech81]

Follow-up to #1: Some flights perform a landing+takeoff, or opposite, without going offline. This is not identified right now by dump1090 nor by the dbmanager module.
Alex

[Xenotech81]

Modification:
After ec79b59 several events for same flight are identified, but which can lead to excessive landing and takeoff identification, if the airplane "jumps" a few times during the landing, as seen for VLG53BQ:

The Docker log shows events within 1sec:

This landing flight is saved as both takeoff and landing flight (the map shows both, but takeoff on top):

• Allow only one event within a short time limit (eg 2sec)
• Clean the database by scanning for landing and takeoff events within a very short time period (<5sec) and merge them