kafka/server: key offset_store records from the group it holds

The offset store embeds a group_id, but a subset of record writers
took an ID off the group and passed that into these methods. The
group_ids match by construction, so we can just use the one embedded
in a group's store.

A notable exception is handle_txn_offset_commit, which gets its ID
off the wire, so we want to respect whatever is in the request. In
this case, add an assertion as a backstop since a violation implies
a logic error in the group routing code.

Signed-off-by: Oren Leiman <oren.leiman@redpanda.com>