Fix issue: CI_DB_postgre_driver::escape_str() cause double _like_escape_chr

20% will be output 20!!%
Change CI_DB_postgre_driver::_like_escape_chr from '!' to backslash "\\"

due to CI_DB_postgre_driver::escape_str() can not append "ESCAPE '$_like_escape_chr'" to the return string,and backslash is the standard escape char for LIKE statement in postgres.
so '20%' will be transfer to '20\%%' through _like_escape_char()