[Chronik] Remove lifetime from MemberSer in Group

Summary: Serializations use owned types like Bytes or [u8; 32] anyway, so it's currently unused, and removing the lifetime allows us to directly store serialized objects e.g. in a cache.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15617