Auto merge of #2940 - str4d:nu-activation-mempool-expiry, r=str4d

Mempool improvements, branch ID awareness

Whenever the local chain tip is updated, transactions in the mempool which commit to an
unmineable branch ID (for example, just before a network upgrade activates, where the
next block will have a different branch ID) will be removed.

Includes commits cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#6654
  - Only the mempool index change.
- bitcoin/bitcoin#6776
- bitcoin/bitcoin#7020
- bitcoin/bitcoin#6915

Part of #2074.

qa/pull-tester/rpc-tests.sh

@@ -28,8 +28,9 @@ testScripts=(
     'rawtransactions.py'
     'rest.py'
     'mempool_spendcoinbase.py'
-    'mempool_coinbase_spends.py'
+    'mempool_reorg.py'
     'mempool_tx_input_limit.py'
+    'mempool_nu_activation.py'
     'httpbasics.py'
     'zapwallettxes.py'
     'proxy_test.py'