sql: fix index consideration with INDEXED BY clause

Accidentally, number of indexes to be considered during query planning
in presence of INDEXED BY is calculated wrong. Instead of one (INDEXED
BY is not a hint but requirement) index to be used (which is indicated
in INDEXED BY clause), all space indexes take part in query planning.
There are not so many tests checking this feature, so unfortunately this
bug was hidden. Let's fix it and force only one index to be used in QP
in case of INDEXED BY clause.