LandsatHlsRaster::findRasters needs to be optimized, remove mutex

[elidwa]

_findRasters in the base class (GeoIndexedRaster) uses multiple finder threads to search for rasters containing a point. In the Landsat dataset code, an SR dictionary is currently used to store all valid band names ("B09", "B01", etc.). However, access to the dictionary must be guarded by a mutex, which limits the performance of multiple raster finder threads.

There are 15 bands for L8 and 18 for S2, so replacing the dictionary with an std::vector or array will have minimal impact on retrieval time for each band.

TODO: Replace the dictionary of valid bands in LandsatHlsRaster with an std::vector to allow concurrent access by multiple threads without locking, and remove the mutex.

[jpswinski]

@elidwa, nominally, reading a dictionary does not need to be guarded by a mutex - only if the dictionary is changing while it is being read (e.g. entries are being added or deleted)

[elidwa]

Fixed this in #433
I replaced SR Dictionary with std::unordered_map<std::string, bool> without changing of what goes into the dictionary/map. Mutex is no longer required. Only main thread created the dictionary and worker threads were reading from it but without mutex code was crashing.