gh-2046, optimized comparators for sequential keys, review fixes

* fix a bug with partial initialization of tuple_format
  in case of zero field count (space with no indexes, we still may
  allocate a tuple for this space, so need an initialized format,
  but will fail to insert it)
* replace assert() for small field_map_size with an error
  message, introduce a new error code for field count limit,
  i.e. limit on how many fields can participate in indexes
* renames & comments