Hold mempool.cs for the duration of ATMP.

This resolves an issue where getrawmempool() can race mempool
notification signals. Intuitively we use mempool.cs as a "read
lock" on the mempool with cs_main being the write lock, so holding
the read lock intermittently while doing write operations is
somewhat strange.
This also avoids the introduction of cs_main in getrawmempool()
which reviewers objected to in the previous fix in #12273

Author: TheBlueMatt

src/validation.cpp

@@ -547,6 +547,7 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
     const CTransaction& tx = *ptx;
     const uint256 hash = tx.GetHash();
     AssertLockHeld(cs_main);
+    LOCK(pool.cs); // mempool "read lock" (held through GetMainSignals().TransactionAddedToMempool())
     if (pfMissingInputs)
         *pfMissingInputs = false;
 
@@ -581,8 +582,6 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
 
     // Check for conflicts with in-memory transactions
     std::set<uint256> setConflicts;
-    {
-    LOCK(pool.cs); // protect pool.mapNextTx
     for (const CTxIn &txin : tx.vin)
     {
         auto itConflicting = pool.mapNextTx.find(txin.prevout);
@@ -623,15 +622,12 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
             }
         }
     }
-    }
 
     {
         CCoinsView dummy;
         CCoinsViewCache view(&dummy);
 
         LockPoints lp;
-        {
-        LOCK(pool.cs);
         CCoinsViewMemPool viewMemPool(pcoinsTip.get(), pool);
         view.SetBackend(viewMemPool);
 
@@ -670,8 +666,6 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         if (!CheckSequenceLocks(tx, STANDARD_LOCKTIME_VERIFY_FLAGS, &lp))
             return state.DoS(0, false, REJECT_NONSTANDARD, "non-BIP68-final");
 
-        } // end LOCK(pool.cs)
-
         CAmount nFees = 0;
         if (!Consensus::CheckTxInputs(tx, state, view, GetSpendHeight(view), nFees)) {
             return error("%s: Consensus::CheckTxInputs: %s, %s", __func__, tx.GetHash().ToString(), FormatStateMessage(state));
@@ -768,7 +762,6 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         // If we don't hold the lock allConflicting might be incomplete; the
         // subsequent RemoveStaged() and addUnchecked() calls don't guarantee
         // mempool consistency for us.
-        LOCK(pool.cs);
         const bool fReplacementTransaction = setConflicts.size();
         if (fReplacementTransaction)
         {