Parallel slave speedup very limited when log_slave_updates=OFF

Branching this off from MDEV-15173 (see that MDEV for all the details)

Parallel slave (slave_parallel_mode=conservative does not provide a lot of speedup over single-threaded slave when log_slave_updates=OFF.

When log_slave_updates=ON the performance is acceptable.

Summary of the investigations from MDEV-15173:

The reason is that with log_slave_updates=ON the server employs XA between the storage engine the binlog:

• Prepare calls sync their effects to disk
  • They are are done in parallel, so they can take advantage of the group commit.
• Commit calls are done one-after-another in order to commit in the master's binlog order
  • but they dont sync to disk and so are fast.

With log_slave_updates=OFF :

• XA is not used (as MyRocks is the only participant)
• commit() calls are done one-after-another in the binlog order
• The storage engine is instructed to sync.
• The above two together mean that MyRocks' group commit does not function.