sql: Enforce scalar type for all indices

Due to the way LIKE operator is implemented, TEXT indices are looked up
with BLOB keys.

LIKE applies both to TEXT and BLOBs, hence a code is generated to lookup
a TEXT and then a BLOB even if column affinity is TEXT.

sql: btreeMoveto() should use MsgPack unpacker.

     * sqlite/src/btree (btreeMoveto): All data is stored in MsgPack
     format. This is yet another place where SQLite native format decoder
     is invoked for MsgPack data. Call MsgPack decoder instead.

sql: Fix some warnings and remove dead code

sql: Fix IdxKeyCompare once more

 * Incorrect format structure attributed was consulted for the field
   count.

 * Outdated formats were handled incorrectly - TUPLE_OFFSET_SLOT_NIL
   doesn't necessary designate a field immediately following the
   previously processed one.