Eliminate mi_key and mi_data from match iterator structure

Eliminate mi_key and mi_data from match iterator structure
- These are not used to hold iterator state but just for temporary
  storage. Temporary data belongs to temporary variables.
- Assimilate rpmdbGrowIterator() into rpmdbExtendIterator() which
  was just (ab)using mi_key for passing arguments into rpmdbGrowIterator()
  and pass as argument instead.
- Declare local structs + pointers to them for this step to keep changes
  minimal and "obvious"