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This fixes a subtle bug involving block re-orgs and non-standard transactions.

Start with a block containing a non-standard transaction, and
one or more transactions spending it in the memory pool.

Then re-org away from that block to another chain that does
not contain the non-standard transaction.

Result before this fix: the dependent transactions get stuck
in the mempool without their parent, putting the mempool
in an inconsistent state.

Tested with a new unit test (adapted for 0.10).

Rebased-From: ad9e86dc
Github-Pull: #5945