buf_page_get_zip(): Do not perform a system call inside a
memory transaction. Instead, if the page latch is unavailable,
abort the memory transaction and let the fall-back code path
wait for the page latch.

buf_pool_t::watch_remove(): Return the previous state of the block.

buf_page_init_for_read(): Use regular stores for moving the
buffer fix count of watch_remove() to the new block descriptor.

A more extensive version of this was reviewed by Daniel Black
and tested with Intel TSX-NI by Axel Schwenke and Matthias Leich.
My assumption that regular loads and stores would execute faster
in a memory transaction than operations like std::atomic::fetch_add()
turned out to be incorrect.