message: Switch CTxMempool::mapTx to use a hash index for txids
diff: #include "boost/multi_index_container.hpp"
  #include "boost/multi_index/ordered_index.hpp"
  +#include "boost/multi_index/hashed_index.hpp"
  class CAutoFile;
  @@ -325,7 +326,7 @@ public:
  CTxMemPoolEntry,
  boost::multi_index::indexed_by<
  // sorted by txid
  - boost::multi_index::ordered_unique<mempoolentry_txid>,
  + boost::multi_index::hashed_unique<mempoolentry_txid, SaltedTxidHasher>,
  // sorted by fee rate
  boost::multi_index::ordered_non_unique<
  boost::multi_index::identity<CTxMemPoolEntry>,
  