The FTS optimizer thread made a false assumption that time(NULL)
is monotonic. The system clock can be adjusted to the past,
for example if the hardware clock was drifting to the future,
and it was adjusted by NTP.

fts_slot_t::interval_time: Replace with the constant
FTS_OPTIMIZE_INTERVAL_IN_SECS.

fts_slot_t::last_run, fts_slot_t::completed: Clarify the
documentation.

fts_optimize_get_time_limit(): Remove a type cast, and
add a FIXME comment about domain mismatch.

fts_optimize_compact(), fts_optimize_words(): Limit the time
also when the current time has been moved to the past.

fts_optimize_table_bk(): Check for wrap-around.

fts_optimize_how_many(): Check for wrap-around, and remove the
failing assertions.

fts_is_sync_needed(): Remove a redundant call to time(NULL).