feat (Db.php) introduced statment cache functions and link to cache class
feat (StatementCache.php) new class to handle statement cahe
fix (Statement.php) free statement resource from application side

oci8 dont properly free resource from application side, but from oci8 side all is normal
refactor (Statement.php) move all flags to one state, introduce state constants
feat (StatementCache.php) return Generator from clean method
feat (StatementCache.php) item bubbles on top of cache when get() is used, new method to get top cache element
refactor (Db.php) implement cache lookup, move lookup logic to separate method, remove lastStatement property