[mempool] Mark mempool import fails that were found in mempool as 'al…

…ready there'

Summary:
I was investigating the reasons for failed imports in mempool and noticed that `LoadMempool()` and `pwallet->postInitProcess()` (for all wallets) are executed concurrently. The wallet will end up importing transactions that `LoadMempool()` later tries to import; the latter will fail due to the tx already being in the mempool.

Backport of Bitcoin Core PR11062
bitcoin/bitcoin#11062

Test Plan:
```
make check-all
```

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4266

src/validation.cpp

@@ -5873,6 +5873,10 @@ bool LoadMempool(const Config &config, CTxMemPool &pool) {
             } else {
                 ++expired;
             }
+
+            if (ShutdownRequested()) {
+                return false;
+            }
         }
         std::map<uint256, Amount> mapDeltas;
         file >> mapDeltas;