Support unclaimed refunds on a separate table

[kderme]

Unclaimed refunds are created when an address is found unregistered at the time of distribution. This can happen for pool deregistrations or dropped proposals. DBSync has access to these refunds and logs them, but doesn't store them anyway. It may be good to have acces to them through a separate table.

This table will only be populated optionally and will be disabled by default. It must be made clear in the config documentation, that by enabling it when db-sync is already synced won't bring back historic unclaimed refunds