refactor unpack_row

Online alter and replication paths differ quite noticeably. It's time to
fix the mess. Online alter has to unpack the old table's data
(here conv_table), and then convert, while replication requires more
sophisticated track: handle possible conversions, treat extra replica
fields, and carefully deduce master's record length.

* Extract unpack_field.
* handle the unpacking progress through a state machine presented by
  Unpack_record_state struct.
* Stick most of the online alter unpacking together under a single
  conditional branch.