Keep mempool consistent during block-reorgs

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.

src/Makefile.test.include

@@ -50,6 +50,7 @@ BITCOIN_TESTS =\
   test/hash_tests.cpp \
   test/key_tests.cpp \
   test/main_tests.cpp \
+  test/mempool_tests.cpp \
   test/miner_tests.cpp \
   test/mruset_tests.cpp \
   test/multisig_tests.cpp \