Memory fixes in cql_connection_impl, some small refactoring

- removed cyclic references in cql_connection_impl.hpp (now cql_connection_impl and cql_session_impl object instances are deleted correctly):

- changed cql_session_impl:_trashcan: shared_prt -> scoped_ptr

- changed cql_trashcan_t:_trashcan:   shared_prt -> simple reference (not necessary cross reference removed)