Get rid of TSRMLS in kernel/string wherever possible

Plus:
  * do not use MM in Phalcon\Escaper::escapeUrl, Phalcon\Text::camelize, Phalcon\Text::uncamelize
  * bug fix for phalcon_fast_strpos() when `needle` is empty
  * fix too early return in Phalcon\Cache\Backend\Xcache::queryKeys