fts_state_t, fts_slot_t::state: Remove. Replaced by fts_slot_t::running
and fts_slot_t::table_id as follows.

FTS_STATE_SUSPENDED: Removed (unused).

FTS_STATE_EMPTY: Removed. table_id=0 will denote empty slots.

FTS_STATE_RUNNING: Equivalent to running=true.

FTS_STATE_LOADED, FTS_STATE_DONE: Equivalent to running=false.

fts_slot_t::table: Remove. Tables will be identified by table_id.
After opening a table, we will check fil_table_accessible() before
accessing the data.

fts_optimize_new_table(), fts_optimize_del_table(),
fts_optimize_how_many(), fts_is_sync_needed():
Remove the parameter tables, and use the static variable fts_slots
(which was introduced in MariaDB 10.2) instead.