Skip saved cache directory when checking SSTables at startup

The SSTable StartupCheck looks for all files which end with "*.db" and compares the version. This causes problems if saved_cache_directory is a subdirectory of a data_file_directories. We should make sure that we are not checking any subdirectory where we might be writing *.db files.

This is the cause of not being able to restart in CASSANDRA-10821.