Reduce likelihood of contention in transactions

[BenjaminPelletier]

Currently, there are circumstances when many transactions may fundamentally contend to change the same data fields. Specifically, if there are many flights being updated in the same area at the same time, the notification indices for each flight's associated subscription will need to be updated for each change to a flight in the area. We can reduce the likelihood of a deadlock or poor database behavior in two ways:

1. Always sort ID parameters before passing them in a SQL command since they will be evaluated in series rather than parallel
2. Add a "for update" flag when reading database entities to use a SELECT FOR UPDATE SQL command where appropriate