Fallback call if upstream share not found properly

Implemented a fallback method in case no upstream share can be found for
a block. This will result in same strange behaviour especially if a
later block has properly added a valid share and this will be used for a
previous block. At least now even the last block will be properly found
and marked as discovered by a user, even though no actual upstream share
was involved in this.

This is a dirty workaround for pools having payout issues. After all
blocks are processed and assuming upstream shares continue to work as
expected, this will *skip* broken blocks/shares.

Workaround fix for #392