Some memory transactions are unnecessarily complex

The MDEV-26769 implementation of memory transactions may invoke a system call within a memory transaction if a page latch is not available in buf_page_get_zip().

In the Intel TSX-NI implementation (CPUID flag rtm), system calls will lead to the memory transaction being aborted, and there should be no correctness problem. The POWER v2.07 Hardware Transactional Memory allows memory transactions to execute system calls with some limitations, but our implementation assumes that no system calls will be executed as part of a memory transaction.

Furthermore, many memory transactions contain read-modify-write operations, such as std::atomic::fetch_add(). Inside memory transactions, plain loads and stores would work equally well and could result in more efficient code.