Limit the number of orphan blocks

In case the total number of orphan blocks in memory exceeds a limit
(currently set to 750), a random orphan block (which is not
depended on by another orphan block) is dropped. This means it will
need to be downloaded again, but it won't consume memory until then.
Merge pull request #3609 from sipa/limitorphanblocks

Limit the number of orphan blocks in memory