WL#1173: better cost estimation in find_best for Full-Text

Affects: Server-4.1   —   Status: Un-Assigned   —   Priority: Medium

Optimizer should not ALWAYS prefer full-text index.

First, index search cost is not 0 in boolean mode.

Second, in NL mode only index search cost is 0, but not row retrieval cost. And
the latter can easily be much larger than total cost of using some other - high
selectivity - index.