Move validate_referenced_accounts call to block/push level

Previously, validate_referenced_accounts was called during transaction
application; however, account creation is one of those changes that
doesn't really take effect until the end of the block, so we want to
validate the referenced accounts for all transactions at the beginning
of the block so that if trx A creates account joe, then trx B references
joe, that reference will fail if A and B are in the same block. We also
call it during push_transaction, as we do want to check it for pending
transactions that aren't in a block yet.

libraries/chain/chain_controller.cpp

@@ -246,6 +246,7 @@ ProcessedTransaction chain_controller::_push_transaction(const SignedTransaction
       _pending_tx_session = _db.start_undo_session(true);
 
    auto temp_session = _db.start_undo_session(true);
+   validate_referenced_accounts(trx);
    check_transaction_authorization(trx);
    auto pt = _apply_transaction(trx);
    _pending_transactions.push_back(trx);
@@ -331,6 +332,7 @@ signed_block chain_controller::_generate_block(
       try
       {
          auto temp_session = _db.start_undo_session(true);
+         validate_referenced_accounts(tx);
          check_transaction_authorization(tx);
          _apply_transaction(tx);
          temp_session.squash();
@@ -441,8 +443,10 @@ void chain_controller::_apply_block(const signed_block& next_block)
    
    for (const auto& cycle : next_block.cycles)
       for (const auto& thread : cycle)
-         for (const auto& trx : thread.user_input)
+         for (const auto& trx : thread.user_input) {
+            validate_referenced_accounts(trx);
             check_transaction_authorization(trx);
+         }
 
    /* We do not need to push the undo state for each transaction
     * because they either all apply and are valid or the
@@ -524,7 +528,6 @@ try {
    validate_expiration(trx);
    validate_uniqueness(trx);
    validate_tapos(trx);
-   validate_referenced_accounts(trx);
 
 } FC_CAPTURE_AND_RETHROW( (trx) ) }