mempool:  convert orphansByPrev from a list to a map.
mempool:  convert orphansByPrev from a list to a map.

This is more efficient and prevents duplicate entries which can
lead to no-longer-orphans being attempted be added to the mempool
multiple times.

Bug found by me, debugged with @davecgh, and patch from @davecgh

qchain/btcd,qchain/btcd