mining: Fix duplicate txns in the prio heap.
mining: Fix duplicate txns in the prio heap.

This prevents the ability for duplicate transactions to be added to the
mining priority heap when a transaction spends multiple outputs from the
same input transaction by converting the dependency tracking to use a
map keyed by the transaction hash instead of a linked list.

decred/dcrd,jrick/dcrd,decred/dcrd,jrick/dcrd