WL#2569: Replace trees with hashes in Full-Text Search

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

Currently the full-text search engine uses TREE (tree.c) in a few places.
This implies O(log N) access time.
Replacing trees with hashes may help here.
Using move-to-first hash optimization may help even more, (and there's no
concurrent accesses here)