Replace `rpc::CoreTxIter` with `list_transactions` fn.

Replace `rpc::CoreTxIter` with `list_transactions` fn.

This fixes a bug where `CoreTxIter` attempts to call `listtransactions`
immediately after a tx result is filtered (instead of being returned),
when in fact, the correct logic will be to pop another tx result.

The new logic also ensures that tx results are returned in chonological
order. The test `test_list_transactions` verifies this. We also now
ensure that `page_size` is between the range `[0 to 1000]` otherwise an
error is returned.

Some needless cloning is removed from `from_config` as well as logging
improvements.