WL#2535: smarter "index merge" in fulltext

Affects: Server-5.1   —   Status: Complete   —   Priority: Medium

In fulltext index all entries are stored, sorted by <word, docid>.
Considering a query "+word1 +word2" fulltext search works like this
(in boolean mode, of course)

  find first (the smallest) docid for "word1" - say, it's docid1
  find first (the smallest) docid for "word2" - say, it's docid2
  repeat {
    if docid1==docid2 then report a match
    if docid1 < docid2 then get next docid for word1
                       else get next docid for word2
  }

(in fact, it's implemented with priority queue and works for any number of words)

The above should be changed to

    if docid1 < docid2 then
      get next docid for word1 that is greater or equal than docid2