[mempool] Allow one extra single-ancestor transaction per package

This implements the proposed policy change from [1], which allows
certain classes of contract protocols involving revocation
punishments to use CPFP. Note that some such use-cases may still
want some form of one-deep package relay, though even this alone
may greatly simplify some lightning fee negotiation.

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html

test/functional/test_runner.py

@@ -157,6 +157,7 @@ BASE_SCRIPTS = [
     'rpc_invalidateblock.py',
     'feature_rbf.py',
     'mempool_packages.py',
+    'mempool_package_onemore.py',
     'rpc_createmultisig.py',
     'feature_versionbits_warning.py',
     'rpc_preciousblock.py',