fts_cache_t::total_size_at_sync: New field, to sample total_size.

fts_add_doc_by_id(): Invoke sync if total_size has grown too much
since the previous sync request. (Maintain cache->total_size_at_sync.)

ib_wqueue_t::length: Caches ib_list_len(*items).

ib_wqueue_len(): Removed. We will refer to fts_optimize_wq->length
directly.

Based on mysql/mysql-server@bc9c46bf2894673d0df17cd0ee872d0d99663121