Revised share group record schemas

In KIP-932, the group coordinator does not persist assignments for share groups. While this sounds like a good idea in terms of minimising overhead for data which doesn't strictly need to be recoverable, it significantly adds to the complexity of working with the coordinator framework.

This issue revised the definitions of the share group record schemas following more closely the schemas used for consumer groups, and eliminating the need to maintain soft state alongside the group coordinator's timeline structure.