WL#2572: avoid unnecessary row reads in Full-Text Search

Example:

in SELECT COUNT(*) WHERE MATCH ...

MySQL can avoid reading data rows, and immediately return the number of found
documents. (and it can skip qsort by relevance too)

the main idea is to set the bitmap of used fields correctly - it means fields
mentioned in MATCH should NOT be added to the bitmap, unless the search does not
use the index.

Note - it is entirely possible to get empty bitmap (all fields are unused) for a
non-trivial query. One needs to verify that MySQL behaves correctly in this case
- skips row reads completely.

For SELECT COUNT(*) we should try to skip qsort() in NL mode - the third
argument of the init_ftfuncs() should take this into accound.