erasure-code: move to ErasureCode::decode_concat

It does not belong to ErasureCodeInterface which is supposed to not have
any implementation.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: rework ErasureCode*::parse methods

The ErasureCode::parse virtual function is derived in
ErasureCode{Jerasure,Isa}. It is reworked to share parsing arguments the
various techniques have in common. The logic is otherwise unmodified.

http://tracker.ceph.com/issues/8496 Refs: #8496

Signed-off-by: Loic Dachary <loic@dachary.org>