Restoring InnoDB buffer pool dump is single-threaded for no reason

Loading the InnoDB buffer pool at startup is only using a single thread. This can be easily confirmed by starting the server and checking that perf top -H only shows one thread being active.

There is no reason at all to pass the parameter sync=true to buf_read_page_background(). If we allow buf_load() to submit asynchronous reads, multiple page checksums will be computed in parallel by the read completion threads while further pages are being read into the buffer pool.