[BUG] Check the job done, when all build index task finished,

[JinHai-CN]

Currently, once one build index task is finished. A condition variable notification will be sent to building job to check if job is done.

We should notify building job, when all build index task is finished but not one task.

In BuildIndexJob::BuildIndexDone function, before send notify_all out, we need to check if to_index_files_.empty() is true. Similar solution can be found in SearchJob::SearchDone.