Rational: Normally calling synchronize multiple times does not matter. However, it can time out if the cache is in use (e.g. saving) and create an exception. This exception is just unwanted "noise".
* search among airline aircraft
* improved logging information
* search among airline ICAO aircraft (which aircraft does the airline use?)
* check ecosystem before merging with VATSIM data
* multi format compatible (automatically detect format)
* flag if cache values shall be overridden
* init from resource files (those are the files coming with the installer)
* automatically read in background if reader is (already) in its own thread (otherwise cache.set() ASSERT)
* new read state "skipped"
* renamed to "isNetworkConnectedAndAccessible"
* checking before reading, which would only fail if network is not accessible
* added amit functions in readers
* cacheHasChanged functions (needed because data of caches are loaded in background)
* pinned small caches
* CDatabaseReader::readInBackgroundThread uses admitCaches now
(applied after 1st performance fix for cache was ready)
* utility functions to detect if only caches are used
* renamed some functions
* renamed some db flags and added new ones
* adjustments to use deferred caches
* removed simulator from CAircraftModelSetLoader`s signature as it was not used
* only change current simulator when explicitly set (avoid unintended setting)
* added function to obtain number of elements from model caches
