Track transaction packages in CTxMemPoolEntry

Associate with each CTxMemPoolEntry all the size/fees of descendant
mempool transactions.  Sort mempool by max(feerate of entry, feerate
of descendants).  Update statistics on-the-fly as transactions enter
or leave the mempool.

Also add ancestor and descendant limiting, so that transactions can
be rejected if the number or size of unconfirmed ancestors exceeds
a target, or if adding a transaction would cause some other mempool
entry to have too many (or too large) a set of unconfirmed in-
mempool descendants.

qa/pull-tester/rpc-tests.sh

@@ -57,6 +57,7 @@ testScriptsExt=(
     'invalidblockrequest.py'
 #    'forknotify.py'
     'p2p-acceptblock.py'
+    'mempool_packages.py'
 );
 
 #if [ "x$ENABLE_ZMQ" = "x1" ]; then