Problems about FileSystem and Env/SpecialEnv

The Env interfaces and file system interfaces have been separated after v6.6? And I am encountering some problems when trying to add a new file system.

First, we have SpecialEnv which inherits EnvWrapper, but EnvWrapper doesn't provide constructor with env's file system and the default constructor will initial a file system called LegacyFileSystemWrapper, the LegacyFileSystemWrapper will just call env(and its function) to operate file. So I made this pr https://github.com/facebook/rocksdb/pull/9267

Second, in db_wal_test.cc EnrichedSpecialEnv implements its DeleteFile, the function will be called by LegacyFileSystemWrapper (because LegacyFileSystemWrapper just call env's function) so that it will fail when you want to call you own filesystem.