[wallet] fix zapwallettxes interaction with persistent mempool

zapwallettxes previously did not interact well with persistent mempool.
zapwallettxes would cause wallet transactions to be zapped, but they
would then be reloaded from the mempool on startup. This commit softsets
persistmempool to false if zapwallettxes is enabled so transactions are
actually zapped.

src/init.cpp

@@ -909,6 +909,11 @@ void InitParameterInteraction()
             LogPrintf("%s : parameter interaction: -salvagewallet=1 -> setting -rescan=1\n", __func__);
     }
 
+    // -zapwallettx implies dropping the mempool on startup
+    if (gArgs.GetBoolArg("-zapwallettxes", false) && gArgs.SoftSetBoolArg("-persistmempool", false)) {
+        LogPrintf("%s: parameter interaction: -zapwallettxes=<mode> -> setting -persistmempool=0\n", __func__);
+    }
+
     // -zapwallettx implies a rescan
     if (gArgs.GetBoolArg("-zapwallettxes", false)) {
         if (gArgs.SoftSetBoolArg("-rescan", true))