Default identifier when group size and group member are specified

[aaronjensen]

When using consumer groups, it is essential that identifier is set on the consumer as well as group_size and group_member. We have established a norm of that identifier being "#{group_size}-#{group-member}, which seems to be safe in all situations. That is, if you scale up or down, you get entirely new identifiers. If you return to a previous group size, the position store backtracks to where it was before.

It would probably be worth either defaulting identifier to "#{group_size}-#{group-member} when group parameters are specified, or even augmenting a supplied identifier with it, e.g. #{identifier}+#{group_size}-#{group_member} if all three are supplied.

If done by default, this would be a breaking change so would require careful consideration and a migration strategy.