Problem:
=======
  fts_cache_append_deleted_doc_ids() holds the deleted_lock and tries to
access size of deleted_doc_ids. In the meantime, fts_cache_clear()
clears the sync_heap before clearing deleted_doc_ids. It leads to
invalid access of deleted_doc_ids.

Fix:
===
fts_cache_clear() should free the sync_heap after clearing
deleted_doc_ids.