vinyl: refactor vy_mem_iterator_seek

 - Don't pass iterator_type to vy_mem_iterator_seek and functions called
   by it. Instead pass only a key and jump to the first statement
   following the key according to the iterator search criteria. Turns
   out this is enough for memory iterator implementation.
 - Fold EQ check in vy_mem_iterator_seek to avoid code duplication.
 - Drop vy_mem_iterator_start and use vy_mem_iterator_seek directly.