Move mempool rejections to new debug category

Move mempool rejections to debug category `mempoolrej`, to make it possible
to show them without enabling the entire category `mempool` which is
high volume.

src/init.cpp

@@ -373,7 +373,7 @@ std::string HelpMessage(HelpMessageMode mode)
         strUsage += HelpMessageOpt("-flushwallet", strprintf("Run a thread to flush wallet periodically (default: %u)", 1));
         strUsage += HelpMessageOpt("-stopafterblockimport", strprintf("Stop running after importing blocks from disk (default: %u)", 0));
     }
-    string debugCategories = "addrman, alert, bench, coindb, db, lock, rand, rpc, selectcoins, mempool, net, proxy, prune"; // Don't translate these and qt below
+    string debugCategories = "addrman, alert, bench, coindb, db, lock, rand, rpc, selectcoins, mempool, mempoolrej, net, proxy, prune"; // Don't translate these and qt below
     if (mode == HMM_BITCOIN_QT)
         debugCategories += ", qt";
     strUsage += HelpMessageOpt("-debug=<category>", strprintf(_("Output debugging information (default: %u, supplying <category> is optional)"), 0) + ". " +

src/main.cpp

@@ -4342,7 +4342,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         int nDoS = 0;
         if (state.IsInvalid(nDoS))
         {
-            LogPrint("mempool", "%s from peer=%d %s was not accepted into the memory pool: %s\n", tx.GetHash().ToString(),
+            LogPrint("mempoolrej", "%s from peer=%d %s was not accepted into the memory pool: %s\n", tx.GetHash().ToString(),
                 pfrom->id, pfrom->cleanSubVer,
                 FormatStateMessage(state));
             if (state.GetRejectCode() < REJECT_INTERNAL) // Never send AcceptToMemoryPool's internal codes over P2P