Track failures in fee estimation.

Start tracking transactions which fail to confirm within the target and are then evicted or otherwise leave mempool.

Fix slight error in unit test.

src/init.cpp

@@ -213,6 +213,7 @@ void Shutdown()
 
     if (fFeeEstimatesInitialized)
     {
+        ::feeEstimator.FlushUnconfirmed(::mempool);
         fs::path est_path = GetDataDir() / FEE_ESTIMATES_FILENAME;
         CAutoFile est_fileout(fsbridge::fopen(est_path, "wb"), SER_DISK, CLIENT_VERSION);
         if (!est_fileout.IsNull())