[Qt][Bug] Load the most recent instead of the first transactions

Fixes a comparator bug when loading the initial `walletTxes` vector.
The design intention is to load the 20k most recent transactions, but it
 was loading the first 20k transactions in error, resulting in a
 potentially large gap in the UI's transaction history.

src/qt/transactiontablemodel.cpp

@@ -100,7 +100,7 @@ class TransactionTablePriv
  // txs are stored in order in the db, which is what should be happening)
  sort(walletTxes.begin(), walletTxes.end(),
  [](const CWalletTx & a, const CWalletTx & b) -> bool {
- return a.GetComputedTxTime() < b.GetComputedTxTime();
+ return a.GetComputedTxTime() > b.GetComputedTxTime();
  });
 
  // Only latest ones.