BUG#18233051 - FTS: FAILING ASSERTION: NUM_TOKEN < MAX_PROXIMITY_ITEM

Analysis:
We don't check NUM_TOKEN < MAX_PROXIMITY_ITEM, so the assertion fails.

Solution:
1. Return error if number of token in a phrase or proximity search;
2. Fix related problem in fts_proximity_get_positions.
   the possible combination containing all the words in a proximity search
   can be a large number greater than MAX_PROXIMITY_ITEM.

rb://4686 approved by Jimmy.Yang