Code review comment for lp:~rhettg/gearmand/no-unique

Queuing background jobs without a unique key is fundamentally broken with all types of persistence.

This leads to confusing behavior where jobs are accepted sometimes, but refused other times because of the underlying schema of the persistence layer.

You can see this behavior by using the special (and undocumented ?) "-" unique key, as well as the empty string "".

This change modifies the behavior to require a unique key when using persistence and providing a useful error message.