[api] More robust and delayed Sphinx indexing for clockwork

I haven't notice the "thread: true" in clock.rb that, in some setups, can effectively kill the "protection" introduced in 88890d8ee6503e98bd0d240eea1060df23d0295b. In any case, this alternative implementation introduce a single method that can be handled by delayed_job and that will (re)index and ensure that searchd is running in a secure way. No more conflicts between starting searchd and reindexing.